I am not sure if there was a standard lettering method.
Unfortunately, period photos seldom show the transom and if they do, then of course only in black and white.
Capital letters, sans-serif, arched downwards was usually used.
Here are a few examples.
All these photos were taken fairly shortly after the boat was built.
#446s Alerion

#490s Wanda

#648s Minx

#711s Ventura

#725s Resolute
