After looking at the two examples of the Mervos-marked watches imported to the US by Horowitz and Son, New York (a 769 on eBay and a 734 at Farfo), I started to think that this was a way Breitling got watches to the US prior to the founding of Breitling Watch Corp. of America in July 1947. I think the serials are all 1944-1946 range.
Belmar was a trademark of Louis Sickles, Philadephia, and Welsbro was a trademark of Weissman Watch Co., New York. All these cases look like the correct Breitling cases matched to the correct Breitling reference number. Of course, the Belmar 790 has a poorly done re-dial, but that case matches the other examples I have seen from 1945.
I think that practice must have stopped after July 1947, except for Wakmann, for which we see catalogs showing Chronomats and other Breitling models with "Wakmann" on the dial (I've never seen one in the wild, only in the circa 1950 catalog).
For the record, Breitling America seems to be older than Wakmann Watch Company. My Dunn and Bradstreet report on Wakmann shows it was established on 31 October 1947. Since Icko Wakmann put up the money for Breitling Watch Corp. of America, perhaps the point is moot. But, I think that it might be that Breitling made watches marked specifically for US importers prior to the founding of Breitling America and afterwords did it just for Wakmann, and maybe just for a few years. In all cases, the watches should recognizable as Breitlings from the Breitling dial patterns, Breitling cases and reference numbers. In other words, they are spot-on, catalog-true Breitlings except for the signatures of a US importer.
It bothers me, then, that the Belmar 790 doesn't have a US importer's mark and the bridge looks all polished-up new. Maybe I'm not looking closely.
Does it make them any more collectible? I don't think so. Probably less. Don't we want our Breitlings to say "Breitling"?
I think Tom could win an Olympic medal for internet image hunting. Is that a winter sport? He found some good examples that seem to fit this hypothesis.
I may change my mind tomorrow, by the way.