Louis Poulsen ph4/3 lamp - help with age please  

They have a website. Hopefully someone there would be willing to help.

As it so happens, I have a red PH 4/3 hanging above my stair landing right now. Attached are photos of the condition I found it in (it did clean up nicely). My version has the loose disk on the top, but I'm not sure how old it is. (Edit: based on the original hanging cloth cord, I would say it's not a recent version.) Not sure if I've ever seen the single formed top before.
Both mine and the eBay fixture are missing the red metal disk reflector in the bottom oculus.
On a related note, one of the key identifying differences between a PH 5 and a PH 50 is that this disk is frosted glass in the latter to allow more diffuse light to fall directly below.

I should correct a statement made above. As per the Louis Poulsen site, the frosted glass was added in 1994 as one of the modifications for the PH 5 plus. The PH 50 update was issued in 2008 for the 50 year anniversary of the model. Therefore frosted glass can be found on later PH 5 examples, in addition to all PH 50 examples.

Just got around to checking my somewhat recent PH 5, and the top has the single piece top. So I guess the 4/3 you are looking at is more recent.