Anyone know specifically where the three feet is located when comparing the early models?
251 vs 281 and 253 vs 283
Cabin? Cockpit? Engine Bay?

I'm betting all of the overall length is located from the dash forward.

You're probably right. Thinking about it now, a 28' would have a stubby look if it had the same distance bow to helm as a 25'

Actuality checkmate played some numbers games length the 30 was actually the 28 hull plus the swim platform was bolted on so they called it a 30 for a few years. The the 281 was the single engine. All 28/30 had 8' beams however, checkmate 251 versus 253 the 251 was the narrow beam, Check out this website for info
https://checkmate-boats.com/