Identical versions. I know both the black and the grey.

Even the Logitech website does not really answer this question. If you compare the "normal" Master 3 there with the Mac version, arbitrarily placed ticks seem to imply differences without any mention of this in the product description. For me, the matter is absolutely not clear. I own a grey version that is not marked "for Mac", but it works perfectly with my Mac, as always.
Is it possible that only the colour with the "Space Grey" is the difference, and the USB receiver is omitted because all Macs have Bluetooth?

I can only answer from the MX Master for Mac. The one with the grey top and the one with the black top are identical. The grey one is simply in the Mac look.