You may be thinking of this C&J Riviera dovetail, Allan. I have it in both configurations, but had never seen the horsehead mold chip before. I believe it is blank on the reverse but have not removed mine from the metal holder. I believe James has and his was blank. I believe M is about right for the C&J chip.

The horsehead mold is (apparently) much scarcer. I'd put it in the $300-350 range in terms of what I'd pay.