[SML] How to gauge the age of a Source Four?

[*Hobbit*]

A vaguely related question, which I don't think I asked before ...
I've found that some of the older 36-degree s4 units in particular were
really hard to get focused right, like the optics were just screwed up
somehow and beam edges always looked ugly no matter what I did ... and
that included lamp optimization on the fly, which I got in the habit of
doing routinely because I could usually get a lot more brightness out
of every unit.  But the older 36es, usually units *without* the 750
model caps, were just constantly balky and ugly and dim by comparison.

I think I remember seeing yellowed reflectors in some of them, but that
wasn't the only problem.  The lens train just didn't seem designed right.
Later units improved somewhat, and I think the lens elements got moved
around in the later barrels but I'm not dead sure about that.  If anyone's
got lens-tube placement graphics from different eras, that would be an
interesting thing to compare.

_H*