I have a Minolta Spotmeter F which I bought second hand a few
years ago, without the lens cap. I would like to buy a lens cap
for it. The manual does not mention which diameter should I buy.
I have to know as I don't think this stuff is so easy to find in
a shop.  Measuring the diameter with a rule I find the outer
diameter is probably 39mm (should use a calibre to be sure).  I
suppose a generic lens cap with diameter 39mm should go, but I
consider that usually lens caps are actually indicated with the
thread filter mount diameter, which is on the inner side of the
barrel. So a cap which is meant to fit a 49mm lens thread is
actually a bit larger than that, as it has to cover also the
thickness of the metal.  So the question is: would a generic
lens cap sold as 39mm diameter work?  something like this:
although I would go for something cheaper.