Your P-14 #448 is, I guess, one of the first Mk I Potters. The Mk I hull is the same as the gunter hull except that the interior is fiberglass instead of wood, but unlike the Mk II hulls there was still wood here and there as reinforcement. If the hole that you cut for the inspection plate was on the centerline of the boat you probably hit the piece of wood that went from the front end of the centerboard trunk to the bow, just under the v-berth. Does your boat have a gunter rig (that is, is the mast wood and does it step through the cabin top onto the v-berth)? If so, the mast steps on this piece of wood, also.
There are two partial bulkheads under the v-berth in a gunter (and I assume also in a Mk I), one from side to side across the front end of the centerboard trunk and one about 18" forward of that. You might also have hit that forward bulkhead.
The gunters (and I believe all other P-14/15's) had foam flotation under the v-berth--that's probably the foam that you hit. As long as you cut your hole off the centerline of the boat and can also miss that forward bulkhead, you should be able to miss everything except foam.
Why did you want to have an inspection hole into the area under the v-berth? I'm not saying that it is bad--I'm curious.
Regards
Tom Grimes
P-14 #363 Far Horizon
Muncie, Indiana