Your existing mount definitely has either a custom plate, or a universal plate that goes along with it. If a custom plate, that was, unfortunately, a poor purchase, because they've had the universal bracket for 20+ years for the RPA series of mounts.

You can often find the universal plate for the RPA series of mounts on eBay as a used product. Sometimes it is easier for find just an old mount with the universal bracket like this one...
https://www.ebay.com/itm/134672700312

Found just the universal interface bracket with all four legs...
https://www.ebay.com/itm/134793200471

If it were me, I would pull off the old 'custom' plate from your existing projector and drill some new holes in it and grab some washers and such from Home Depot to get it to fit your new projector. It's way less money to do this, and will be just as secure if you take your time doing it. I've done this a number of times with 'custom' plates that I modified to fit various projectors. Saves a ton of money overall.

There are sometimes new models like the RSMAU on eBay that might be a similarly good deal that gives you the newer Elite style mount with the better universal interface. For example...
https://www.ebay.com/itm/185975704548

What makes the Elite series better is the fact you can micro adjust everything even after it has all been installed and tightened down. It's a very cool system.

Yes, that plate will work with your old RPA mount just fine and is a solid solution. If you ever do get a new model, you will need to pick up an extra arm, which they sell on eBay as well from time to time.

Good luck with the Optoma.