THe RCBS die will have something stamped around the top face - If it is a neck sizer it should have 'NECK' there and a two digit date of manufacture. The Lyman ones should be stamped on the sides by the knurling.
Each manufacturer makes dies to different specifications so if they are both neck sizing dies one may size a bit smaller than the other or one may not be a neck sizing die at all.
No 308 dies to swap - but someone may well have a spare set who wants .270 dies.
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