lots of BMW people talk about matching numbers, even in newer 70's and 80's bikes. if all this hand crafted nonsense is worth anything more then then prestige, it must be subjective. While Hasselblad and lieca make sharp lenses, and were originally an innovative design, no handcrafted thing can compete with machine and computer precision (at least in terms of reliability and consistency) which is probably why hasselblads are now made by fuji, in Japan, on a cell system assembly line (each product is built from start to finish by one person).
If the backs have such tight tolerances so that they must be matched to inserts, it kind of defeats the purpose of having inserts in the first place, that is so that you can have many inserts ready to go instead of having lots of full backs ready to go.
Just the same as selling spare parts for a motorcycle, where you cannot really replace any parts, because they will never match or work properly. With older BMW's it is the fact of life that while they are fairly reliable, they always leak, klunk and woozzie, they are very heavy, and all that for apparent reason, other then tradition... I feel the same way about hasselblads and liecas...