My Pedersoli is pretty much exactly .690, and a Marty's 670 fits perfectly with a little over two wraps of masking paper. I haven't tried it with cloth patches, but I imagine it would need something thin like a .010 patch if not a smaller ball. I have an antique smoothbore that's closer to .700, and with a lyman .662 ball, a .018 patch was almost tight enough to have to pound on the short starter.
Marty's can make moulds that are two different sizes (they're four cavity in this size range) so maybe you can special order one that's half 670 and half 660 or 650. The balls come out great on my 670, but the sprue holes could be a little bigger because a lot of them have the lead harden in the sprue before the cavity is filled.
I don't recommend the lyman 662 unless you can find an old one. I got one where the two halves were offset from each other by almost .010, sent it back under warranty, and the replacement was just as bad. The replacement made earlier this year and the first about a year before. I'm about to sell it for a loss.
