At the last CLA show in Lewisburg, PA, they had a large exhibit of the original rifles also. I had the opportunity to study RCA 53 in detail. There were faint traces of engraving on the BP extension. It consisted of a fine line paralleling each side of each facet of the octagonal flats. At the front end, adjacent to the wedding band these lines were connected with an arc. At the rear end the just ended.
I hope this helps.
Best Regards,
JMC