I've looked at a few Miroku sidelocks, Larry. It seems to be the same gun mechnically, BUT, I'd say the Brownings were built under more strict quality control, and the Mirokus all seem a bit porky by comparison.
The Brownings seem to have more even engraving and higher quality finish, and more consistant weights, gun to gun. Keep in mind it is a slim sample of each, and my opinion, but, that is what I've seen.
In France, I actually saw two 12 gauge BSS sidelocks. They had the required sling swivels fitted, and, I'd guess, it was a factory job.

Best,
Ted