I have just acquired what I believe to be an original 1888 Rod Bayonet TD. It has a Buffington rear sight, single piece trigger guard with integral sling swivel, stacking swivel, and SWP 1891 cartouche. It has an 1884 breech date but a 1880 serial # (140XXX). From what I have researched, Springfield did assemble numerous 1888 models using left-over receivers or ones reclaimed from earlier trapdoors with serial numbers in that range. The rifle has an even patina to it and doesn't look like a parts gun.

My question is, are 1888s marked 1884 on the breech?

Yes.
Mine is the same.