Hans Wegner Johannes Hansen Chair  

That's a cool chair.  Not something I had ever seen before. 

JH 525? Might have another number when it's webbing instead of fabric.

Thanks. I only did the random JH-5XX searches up to 520 before giving up. I thought that a 1951 design would not have so high a model number.
 
It might also be that there was never an original upholstered option for this chair, and that 525 refers to only the caned version. Not only am I guessing that the green plaid is not original, but the middle front rail detail does not seem designed for an upholstered option.

I think you are right. The upholstery does look very home-made.
Incidentally, the picture refers to an Bruun-Rasmussen auction. Bruun-Rasmussen, unlike many others (none mentioned, none forgotten), actually puts som pride in getting designer and manufacturer right. I've never come across any errors at their auctions. So I consider them credible a source.
http://www.artvalue.com/auctionresult--wegner-hans-j-1914-2007-denmar-jh...

Just an update.
I found the picture below, in an old Bruun-Rasmussen catalog. the description states the model as JH 525, and that the design is from 1959.
As you can see, these chairs variete a little, compared to the chairs shown above. f. ex. the piece of wood between the front legs (don't know the english word for it) are thicker on the last chairs.
I still can't find another model no., so maybe the alterations were made, simply to improve the construction/stability of the chair.

That chair drawing (both arm and side versions) is shown in the Wegner book as designed in 1959, but separate from the first chair in this post designed in 1951. In addition to the different angles of the sloped rail-stretchers, the side rails are also different. They are very similar to each other.
Too bad the book gives dates and manufacturer for all the chair drawings, but only model numbers for the select few that get discussed in detail.

I've got a couple of updates for this thread:
First, I did some more research over the past month and think I've got all the model numbers figured out for these Wegner "coat hanger" front rail chairs. Due to similarity of features, the below chairs seem to get mixed up a lot in online listings. I've also included some key ID features to help quickly distinguish each one.
CH-31: Carl Hansen, dining side chair, 1956, cane only, thicker side rails with slot for cane, steeper coat hanger front rail, back legs kinked at seat rail; source: Hans J. Wegner's 100 Chairs, pg 85
JH-515: Johannes Hansen, dining arm chair*, 1951, cane or leather, recessed thin side rails, steeper coat hanger front rail, straight back stretcher, back legs kinked at armrest; sources: Hans J. Wegner's 100 Chairs, pg 182; Grete Jalk [ed.]: "40 Years of Danish Furniture Design", vol. 3, p. 174-175
JH-516: Johannes Hansen, lounge arm chair*, 1951, cane or leather, recessed thin side rails, steeper coat hanger front rail, straight back stretcher, back legs kinked at armrest; sources: Hans J. Wegner's 100 Chairs, pg 182; Grete Jalk [ed.]: "40 Years of Danish Furniture Design", vol. 3, p. 174-175.
JH-525: Johannes Hansen, dining arm chair, 1959, leather, thicker side rails with leather wrapped around rail, shallower coat hanger front rail, shallower coat hanger back rail, back legs kinked at seat rail; sources: Hans J. Wegner's 100 Chairs, pg 185; Dansk Kunsthåndværk, no. 7-8, 1958, advertising pages.
JH-526: Johannes Hansen, dining side chair, 1959, oxhide, thicker side rails with leather wrapped around rail, shallower coat hanger front rail, shallower coat hanger back rail, back legs kinked at seat rail; sources: Hans J. Wegner's 100 Chairs, pg 185; Dansk Kunsthåndværk, no. 7-8, 1958, advertising pages.
Magasin du Nord (no model number, but similar to JH 507): Johannes Hansen, arm chair, 1950's, leather, thinner side rails, shallower coat hanger front rail, low side stretchers, downward inclined armrests, exposed wood back of backrest, back legs kinked at seat rail; sources: Wright, Bruun-Rasmussen, Lauritz auction listings
* Note: Since the Wegner book does not have model numbers for this chair, and I have not yet seen the 40 years book reference (obtained from a Bruun-Rasmussen lot), there is the possibility that I've gotten the dining and lounge chair model numbers backwards.

So why did I decide to waste my time and figure out all of these chair models half a year after I started this thread?
That's where the second update for this thread comes in. Well, I just got my hands on a teak JH-515 with original oxblood red leather. It is unmarked, was listed as simply a "Danish Armchair" in a recent auction, and no one else recognized it as a Wegner/JH piece. A couple of loose joints will require regluing, and there's plenty of grime on the wood, but the frame members are all structurally sound with no breaks/repairs. Will post photos once wood frame has been re-glued and cleaned up. The leather will remain as is.
Very happy to welcome the third Wegner piece to the household, joining a CH-23 and a JH-501. Not as famous as the first two, but much tougher to find.

Gorgeous chair. Nice it has the original leather. I've had good luck with removing original leather, in order to disassemble and re-glue, then putting it right back on. Also makes it so much easier to clean when it is in pieces.

Was just looking through a 1958 Dansk Kusthaandvaerk and came across this add for the CH 31 (nr. 31). Although it does not add any new info to this thread, I figured I'd post the picture

. . . with above mentioned pages from the "40 Years" volume. No model number reference, but photos of JH-515 cane version dining chairs.

While I had finished the re-gluing effort early this year, it took me a little while to figure out what to do with the leather. I ended up simply applying some leather conditioner, which made it much softer but also darkened the color. However, I like the contrast now between the teak and dark red leather. The worn areas at the front pretty much stayed the same. I did not want to try to match color the worn areas to match due to concerns of making it worse.

Looks fantastic! Nicely done. I think you made the right call with the leather. Old leather is simply gorgeous.

... and finally, vintage catalog verification for the four JH models, including confirmation of 515 being the armchair with 516 for the lounge chair.