No problem Glenn. If I had a P51B/C, and it meant getting a canopy made which had to have a thin frame, I don't think I'd be complaining about such a relatively small detail!!
I believe, and Antoni also mentioned this, that one of the reasons for the lack of vertical frames was to allow the canopy to shatter, or break in two, if it hit the fin after being jettisoned, probably to lessen any chance of it colliding with the pilot following it, and causing injury/unconciousness. Also, as the B/C version originally had a substantially framed, hinged canopy, and the rails for the sliding 'Malcolm' hood had to be mounted semi-externally, this would probably have caused difficulties (for ease of production) fitting a frame which would match, and seal against, the windshield frame. The 'rolled edge', on the other hand, could be moulded to 'slot' into the mating surface of the windshield frame quite easily.
EDIT: Crossing posts - no problem Igor, glad to help. Did you see the PM about the MiG kits?