If it is a standard Beselar lens board, the ring is not part of the lens
board and if you turn it upside down, it should fall into your hand or onto
the table.

I can't remember now just exactly how the Besler 23 lense board
works (haven't used one for 20 years), but generally the way
people deal with changing lenses is to have each lense
permanently mounted on a different board.  The entire board +
lense is swapped.

Yes, there is a significant difference between 75mm and 50mm
lenses.  But first, which lenses do you have?  And what size
negatives are you going to work with?

Argh. You shouldn't have to take out any screws to remove the
lensboard. It's spring loaded; you angle it in against the rear spring,
and press until the front edge will fit past the lip and stay in place.
I hope you get it all put back together...

How to make a simple task difficult and destroy your new enlarger all in one
easy step. Follow Scott's comment.  Do not disassemble the enlarger
unnecessarily.  Mosts tools are designed for easy operation.  If you do not find
the easy way, keep looking.

Thanks for the help. Is there any

25 mm.  Seriously, the lenses are sized to the negatives they are intended to
develop.  (By the way, notice that the top rack and pinion bellows control has
markings for the locations that you should stop at for negatives from 35 mm to 2
1/4 x 3 1/4 inches.  You have to adjust this for each lens that you use.)  A 35
mm negative is 24 mm x 36 mm and the diagonal distance is about 43.5 mm.  So a
50 mm lens is large enough with a little extra to "cover" the diameter of the
negative when projected.  A 75 mm lens is about appropriate on the same basis
for a 2 1/4" x 2 1/4" negative, which is about 56 mm to a side.  (SQRT [56^2 +
56^2] = 79 mm.)  In fact, some people prefer a slightly larger lengs for the
job.  I often use a 90 mm lens for a negative this size.  To complete the
analysis for the lenses available for the Beseler 23C, (1) a 90 mm lens is
required for a 6 x 7 negative (nominally 60 mm x 70 mm or 2 1/4" x 2 3/4"), the
negative is about 56 mm x 65 mm, yielding a diagonal of 86 mm; (2) a 105 mm lens
is required for a 6 x 9 (aka 2 1/4" x 3 1/4") negative, with a diagonal of 102
mm (SQRT [56^2 + 85^2].

Francis A. Miniter

Lenses for most enlargers use the standard Leica thread on the mounting
flange. Of course, Leica don't use it on their cameras anymore. As
others have said, the lensboard for the Beseler can be as simple as a
piece of 1/8 " plywood with a 40 mm hole cut in the middle if you have
the retaining ring. I used to get 1/8" plywood at hobby shops.

25 mm?