Ah, live and learn! Thx!
I did not see base stretchers in the first photo and I could have mistaken teak feet for metal. Wedding ersatz legs to upper shelving "hutch" units and advertising them as vintage pieces has become a bit too common among certain dealers near here lately. Perhaps I was too quick to judge.
In zooming up close in the first photograph, I believe your bookcase is an early Søborg production of the Mogensen #128 design model, from the early 50's. The early production offered an option for pine finishes on the inside of the case and shelves, with teak for the external case housing. The shelf fronts exhibit the classic tapered Søborg profile (although upside down currently). The spacing of the pegholes on the sides also looks like Søborg.
If the case dimensions of your bookcase match these 1952 (Danish) 1954 (English)catalog cuts, then it is most probably a legit Mogensen piece for Søborg.
The legs are not original. If the bookcase was mine, I would remove them.


Birch interior was an assumption on my part. If you say pine, that's good enough for me. I'm also assuming "teak/beech, teak/oak, teak/teak" refers to base/leg options, but the 1954 Danish catalog seems to mention a mahogany option? Otherwise, I agree that the legs in @aqeula's photo are likely not original as I had first suspected. There's not a lot of "design" with this piece beyond proportional dimensions and leg/stretcher assembly, but the tapered front edges of the shelves (yes, inverted in the OP's image) are a nice touch!
tk, I noted pine primarily because of the grain patterns I saw. Beech looks different, and I usually associate birch as the cheap light wood US counterpart to Scandinavian beech.
The bookcase housing in indeed quite minimalist. However, the design should be viewed in context within the whole modular system in which it was designed. The lower drawer boxes, secretary desks, and tall/short cabinets all work together to provide a wide range of design options. We were lucky enough to get most of a whole wall's worth of modules from 3 lots in a single auction 4 years ago. A couple of fill-in auctions later, and we were able to assemble a nice wall setup that resembles a customized built-in design.


Birch, beech, pine is almost moot to me anymore. Knowing wood species can be helpful when trying to ID makers but, depending on variety, individual tree, or even a particular board, the difference between the three hardly matters. I do like the color contrast between interior and exterior surfaces of the OP's bookcase(s?), however.
Thank you for photos of your domestic space. Elegant and impressive, I must say! Though, now I suspect you might suffer from a similar "symmetry syndrome" as my now-exclusive client. I've been trying for years to free him from this affliction, but to no avail. As he is a specialist in French Neoclassicism, I fear his case may be terminal.
P.S. How many Quistgaard ice buckets are too many??
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