Picked up a nice recent Apo-Nikkor for about the same as it is costing for SKG to drill my lensboard for the flange (I plan to use it in barrel). Just a little (maybe) problem: there is a small (approx 1mm) object, which I think is inside the rear cell between the two rear (airspaced) elements. (See photo).

Does it look like fungus?

If it is fungus, what to do, if anything? First move was to make sure it's not on the front of the rear cell. I tried to remove the rear cell by holding the front and rear rims of the lens with rubber (bike inner tube) but too tight... it wouldn't unscrew. (Find a way to apply more twist, or wrong approach?).

Have given it a good sunlight soaking. (2 days x 3 hours). But worry that the warming of the whole lens in the sun will have negative effect on diaphragm lube.

I am tempted to just unscrew the rear retaing ring. Have read conflicting advice on this forum on this...centering is determined by the ground edges of the element whose seating is defined by the mount and retaining ring, versus send to technician...the latter not really an option because of cost.

Or do nothing...it's near the edge; and might not get worse in my lifetime.