I was just told P320 slides have at least three different layouts for being milled.

Does anyone know which one will fit the holosun 507C without an adapter plate if any?

Does slide size matter 3.6, 3.9, or 4.7.

I waded into these waters while putting together the pages for the Grayguns custom slides. Please do not take my word as the "final" word, as someone should confirm.

The Holson 507C footprint is the same as the Trijicon RMR. This page indicates they share the same "RMR Mounting Standard."

https://www.optics-trade.eu/bl...osun-507c-footprint/

Grayguns offers slides that are cut to the RMR standard without needing an adapter plate.

Modern Classic 3.9" (In stock today with barrel and guide rod included. There is a slide and slide internals only version out of stock.)

P320 Modern Classic Slide, 5” Bull Barrel (In stock)

P320 Modern Classic Slide, 4.7” Bull Barrel (Out of stock today)

P320 Lockwood Slide, 4.7” Bull Barrel (Out of stock today)

Please read the descriptions carefully. The systems above include a barrel, guide rod and slide internals.

Sig is now selling P320 and P365 OEM slides with optics cuts.

I have seen pics showing 3 different versions of the Optics cut and have no idea yet what to order.

I don't think any SIG slides (from SIG) will fit without an adapter plate for your choice of optic.

Their slides have Romeo1Pro, Romeo2, and/or DeltaPoint Pro setups. Those mentioning the RMR footprint also mention needing a sealing plate.

The RMR footprint seems to be the right one you're looking for for your choice of optic. So the most likely slide that will meet your requirements will be a slide that has a direct fit for the RMR.