[455-4+1]

Pulled down a few engines recently that looked to be pretty original.
On the decks were stamped letters near each bore, possibly designating tolerance post hone?.
S, T etc.
Does anyone have any information on these tolerance stamps, if they are indeed that.
Stamping looks pretty uniform, so may have been machine probed, then stamped, and assembly guy picked a corresponding piston to suit to try and close up tolerance?