Dave,
My thoughts are the same as yours regarding locks. I agree with the earlier posters thinking the gun is an early one, Rev War or so. So the square tail one is too late. The Bess style might work, but I think the round face pointy tail would be more appropriate.
I thought I had a picture with me of a similar era fowler I have, but can't find it, so guess its on the home computer. But, Mike Brooks just posted a pic today on the 'First rifle' thread of a fowler he did. Check out the lock on it. That's style of lock is what I'd shoot for.
Another thing, as far as the bolt hole spacing, and again as earlier posters considered. Try to determine if the gun is a straight english type trade gun/fowler, or, if it was originally made up of new and used parts. If it's a 'parts' gun, you could just plug one or both of the bolt holes in your round faced lock, and redrill them in the right place, since originally it could have been a reused lock anyway!
Also, check your private messages. I going to send you something.
John