The 507c uses the rmr footprint, I believe the 507k uses the delta point/sig pattern.
Yes, from everything I have seen (now) the "C" versions use the RMR pattern. The consensus I have found is the "K" versions use a proprietary modification of the Shield footprint. Below is the best website I have found for info, and a copy/paste that explains it.
I have a Beretta 92X RDO which I guess is still considered "new" to the market, the Holosun "K" series also "new", so I guess things are not caught up quite yet. That and of course everything is floating in the Pacific waiting to be unloaded. Contacted Holosun and Beretta last week about a plate, but have not heard from either.
Beretta gives you a choice of a "free" plate so I put in for a Docter/Noblex footprint which fits a bunch of sights including Vortex and Burris. Not entirely enthused about $50 for a plate from Holosun (if they have one).
All that said, at this point with my increasing frustration, I think I'm going to get a Burris FF3 as they are well proven, considerably less cost, fits the Docter plate I have coming. Maybe a Vortex Venom if I can find a good deal.
https://optics-info.com/footprints-on-red-dot-sights/
A modification of the Shield standard
Important Notes
Holosun 407K and
Holosun 507K feature a modification of the Shield footprint. The differences are:
- the two rear sockets are missing
- the sockets in the front are shallower
For this reason, some pistols that are optic ready for Shield footprint need to be modified for the 407 and 507K. Namely, the two rear indexing lugs on the pistol have to be removed and the two front indexing lugs shortened in height. Hellcat and Walther PPS are examples of such pistols (some pistols, such as Sig Sauer P365XL, do not have to be modified)