Measuring the stroke on a merc motor?

I have a question about identifying a Mercury block, heads have the EAC, so i popped off one of the heads. I stuck a wooden rule down the lower part of the cylinder wall and to the top edge of the block it measures a hair under 4".
piston was completely at the bottom and i was not on the dome. Is the truly a merc motor?

 

yup

 

That will make it 255ci.

 

Thanks for the help!!!