You can use a Mark II mainsail (the sail for the later model P-15's) on P-14's. If you have a gunter rigged Potter (wood spars and a second spar that the sail is attached to, that is hoisted parallel to the mast) you need to attach the smallest size of sail slugs to the leading edge of the sail. The rope along the leading edge of the sail is too large in diameter to fit in the groove of the gunter spar, but small sail slugs will fit.
If you have a P-14 with an aluminum mast with no gunter spar, it may be that the rope on the later model mainsail will fit without using sail slugs, though using them makes getting the sail up and down easier.
Some of the people with P-14's have changed to the later model, larger Mk II mainsails because, being bigger than the original sail, they are better for sailing in light air.
If you have the original gunter rig on your Potter, you will have a problem with the full length top batten on the Mk II sail--when you lower the sail, and the gunter spar comes down parallel to the boom, the long batten in the sail will totally block your vision across the cockpit. Also, it is better to get a Mk II sail that does not have a line of reef grommets in it, because the first reef in a MkII main set on the gunter rig needs to be 24" above the boom, not where the reef is usually positioned for the later model Potter.
While the Mk II mainsail is a bit of a nuisance on a gunter Potter, it is a faster sail, particularly in light air. And I should imagine that they would be more easily obtained than an original gunter sail.
Regards
Tom Grimes
P-14 #363 Far Horizon
Muncie, Indiana