Swap Tessar front cell (glass only)

I have a Contaflex (late model, Super or similar) with damaged (cloudy) coating on front surface of the Tessar 50/2.8. S/N 4004039.
And a Contessa LKE (not The Contessa) where the body has been bent beyond repair, but the front cell of the Tessar 50/2.8 is OK. S/N 3197624.
Both cameras are from about the same era (mid-sixties), and both front elements (glass) seem to have similar diameters, 19mm give or take a few 1/10. Hopefully Zeiss opticians did not re-compute different Tessars for same-era cameras.
I'd like to transplant the good glass into the Contaflex front cell. Both cells seem to have the glass fastened by the front ring (with Tessar, etc engravings) screwed in. But no slots in that ring.
Questions.

1. Can you please confirm that glass is fastened by a slotless screw-in ring?
2. What is the preferred method for un-screwing?
   1. friction from a rubber stopper
   2. as above, but enhance friction with two-sided adhesive or super-glue (will worry about removing glue later)
   3. drill a pair of small holes and use a spanner; ruins the collector value
      
   4. something else??

Alternatively, I could continue the polishing that I have started on the defective coating using Cerium oxide. Some progress, but in the best case, this will result in an un-coated front element, and in the worst case I'll ruin the lens in a different way.

Hi Bernard

I just looked at my front lens from my contaflex and there appears no way to remove the glass from the backside, so it must be removable from the front. I would first try with a friction rubber stopper (needs to be pretty small to access the name ring since the filter size is 27mm). If it is somewhat stubborn, a little acetone around edge and/or a little heat may help loosen it up

Very possible that I am not understanding the issue here, but it sure looks to me that if you could separate the silver ring from the black lens mount #400xxxx, that the black lens mount #319xxxx would go into the silver ring? Is the upper lip of #319 threaded?

For damaged coating, if you want to try removing it you can use the polishing stuff form watches glass polish,like this one:


Polywatch Watch Crystal Glass polishing Paste with W5 Wipe Cleaner

And for swapping lens I prefer to do it as a whole set rather than just a group or an element. Most of the Zeiss 's lenses are paired in original. If my memory is right, I could recall some of them even have the same set of serial numbers both at the front and rear to confirm as a set. The rear s/n could also be hand crafted on the rear group glass barrel.

Thank you all for your input.

@Leolab. That is what I'll try first. I'll use Degrip Oil (naphta/light oil) that from past experience works well for aluminum threads. I have a collection of rubber stoppers. But probably I won't escape drilling small holes.

@Dan Daniel. The silvery metal is just one piece. And when the black engraved ring is separated, the glass will be free. So it needs to be replaced by glass, not by a metal/glass cell. And, the rear thread of the #319 is smaller --22.5mm-- than the front thread of the #400 --26.5mm ID, corresponding to 27mm Contaflex filter thread. And... axial distance.
Maybe not obvious on the picture, but the metal mount of the #319 has a similar two-part structure, the only difference being that the outer part (receptacle) is smaller (and black). Same 27mm thread for filters and 28.5mm outer diameter for push-on Proxars. Possiby I might even swap the #319 retaining ring into the #400 mount.

@kl122002. I'd rather not. This polishing paste is meant to remove scratches, and therefore digs into the glass; From one Amazon customer comment:
Worked well for a very light scratch but initially created a weird splotch in one spot, I had to apply it to the whole face which took about an hour
Cerium oxide is the last step of lens polishing, when the figure of the lens is completed, and all that remains to do is remove micron-scale roughness that separates ground from clear glass.

Update. Executive summary: failure. Detailed version below.

The Contessa LK Tessar mount is in fact one piece. The lens is crimped on the rear rim by a ring of soft aluminium that must have been pressed when fabricated. Using a cutter blade under a low-power binocular microscope, I pushed that aluminium lip outwards and could separate the glass element from the mount. See first pic.
The Contaflex Tessar front element is mounted in a more "normal" way. Drilled two small holes in the front ring to gain a good grip with the lens spanner. See second pic for a group picture of the two Tessars, unmounted. Note the stronger reflection on the front side of the de-coated Contaflex Tessar.

The good news: both front elements have the same diameter: 19.51±0.01mm.
The bad news: they do not have the same edge thickness; the Contaflex one is thicker. So, the Contessa Tessar fits snugly in the Contaflex cell, diameter-wise, but with the holding ring screwed in, there remains some axial play. And, even if the mechanical fixation were good, the difference in thickness would probably mean a change of the infinity setting, which should not be adjusted because compatibility must be preserved with Pro-Tessars. And that is before worrying about loss of optical quality.