Large vintage 4" telescope lens or magnifying glass convex round glass lens

Description

smoke and odor free
enlarge pictures, that helps with description
on close examination I see a fine scratch 1" about 11 o'clock
but you might more
one side the lens measures 4", the other side is 3 3/4"  it is encased in a black hard plastic
the sides show threading but I do not know what it fit in.
There is an opening for a screw ? on each side of the black plastic holder
The only markings are a number in the hard black plastic
holding case  53711
prior to packing this weighs 2lb 6.3oz
please ask questions, willing to provide additional pictures   NO RETURNS
email questions prior to bidding.  Quick careful shipping with tracking the day you pay. Check both methods.
Thank you~
Actual USPS Priority shipping with insurance, I do not add handling.