Checking. Might even try photographing, because there are some reinforcing strips squaring off under the sound hole, almost like what you'd need if you actually replaced the ring and hole or added on.
The rosettes don't go through the wood, they're inlaid in a routed circle(s), so I suspect what you think are reinforcing strips are part of the standard reinforcing bracing around a soundhole, there were varying methods used in dreads over the years but I don't know what an M20 or M30 pattern might have looked like.
Sometimes they're just a plate laid inside the bracing square that typically surrounds the SH, so that still might fit what you describe.
I suspect they used an M30 type top simply because they had it handy and could be cut down to M20 dimensions easily.
(That's assuming Hans' assumption about it being factory work is correct, and he doesn't like to speculate without solid reasons to do so.)
Wouldn't be the first time we've seen a replacement top from a different model:
Saw an F30 with an F112 top once, confirmed by Hans because it had 12-string type bracing and the F112 was based on the F30 body and outline, so obviously they used something handy that fit.
I was going to have Dan Erlewine replace sections of the SH binding on my J50, but when he said he'd just cut out the circle and patch in a new one I ran scared. I just never thought of approaching a repair so radically. I don't see ID markings or notations under the top though.
Not sure I understand, did you mean he actually wanted to cut out a section of the top (Yes that is pretty radical!) or is that possibly what you
thought he meant when he just wanted to replace the rosette rings?
Also we have had reports of replaced tops being signed by the guy who did 'em at Guild but can only recall 2 instances, one for sure (Carlo Greco signed the top when he replaced it on Chaz's F50r IIRC); and another for which the specifics elude me.
Doesn't mean they did it consistently though, the practice might have fallen off as production and repair volume went up.
The F30 mentioned above didn't have a sig IIRC, so lack of a signature doesn't necessarily rule out a factory repair.