This may be a dumb question but it's something I haven't encountered before. Today I bought a well-worn but mechanically sound Model 10-6. On the cylinder crane and on the left side of the grip frame there's a stamping of what appears to be a spade, as in a deck of cards. Any idea what it means? Other stampings on the grip frame include the circled letters RE and, below that, the letters XP. Thanks.