I went through my boxes of seat belt parts and may not have much of anything to help you. As background, when I restored my coupe, I copied Paul Cain and installed three point belts with the take up reel inside the B pillar stub and a swivel point on the roof with a rotating 320i leg mounted to the side of the passenger floor area. This required welding a metal plate in the B pillar stub and modifying a slip ring that bolted to the roof.
As a result my parts have a different buckle and female receptacle then yours. However I found it was easy to change buckles on the belts. Just cut it off with scissors, then do the same with the new belt. Gently flame the cut end of the new belt to melt the nylon together and prevent unraveling. Take the buckle and new belt to a cobbler or upholstery shop and have the old buckle sewn onto the belt. I took a model of the old stiching and found that it could easily be copied. Also some might be concerned the new sewing was not sufficiently strong. But I was always impressed compared with the strength of the original. So I think you could use new belts and take up reels with your old buckle and receiver.
Anyway I am attaching the parts I have. I have two new, unused belts with take up reels 72 11 1 941 391. I am not sure how these compare to your reels or if they would fit in the plastic shroud, but they are new, unused and I don't need them. In the other picture is one belt with the rotating leg which is what I put in my coupe. I believe these are from a 320i. I do remember hunting for 320i seat belt parts when I did the three point conversion. Note that this take up reel is a little smaller. At the time I did the conversion, these smaller reels were no longer available, but I did identify at the time shops that would restore them (primarily update lubrication and respring to regain take up function). I also have number of miscellaneous mounting bolts bushings and springs if by chance needed.
I can post pictures of my B pillar, roof mounting and rotating 320i leg if it is of any use to you. Let me know if anything might be helpful.
