Well, the primary purpose was, as it was on the "military" revolvers, to provide a lanyard attachment point.
ETA: OK, apparently the company referred to it as the skull cracker. I thought that was something that was added later by popular usage. Other M&H revolvers had lanyard rings on a flat butt.
I don't believe military Colt 1873s or Smith & Wesson Schofields had lanyard attachments, which is odd, given that many went to cavalry, but it was a common feature on military handguns of the time.
The 1911 destined for cavalry use had a lanyard loop on the mainspring housing and on the magazine.
As for whether it's pre-1898 or not, I believe that M&H went belly up in 1896, and Hopkins and Allen ceased production of revolvers at that point, so all M&H revolvers should be antiques from a legal standpoint.