In article <4t5nhc$s0c@news.nstn.ca>, chefurka@fox.nstn.ns.ca says...
>I read here some time ago that the glass from an F4 focussing screen
>could be used to replace the glass in an F screen (IOW the glass is
>the same size, so you take the glass out of the metal frames on both
>screens and put the F4 glass into the F frame).
>Does anyone know if this will work with the F3 screens as well -
>can one put the glass from an F3 screen into an F frame?

The "glass" (actually, most of the screens have a glass condenser lens
on top, with a plastic GG and Fresnel lens on the bottom, with a thin dust-leaky tape holding them together) is the same size in the screens
of the F, F2, F3, and F4, and all interchange in the two different
frame types (F/F2 - F3/F4).
Hope This Helps