The info on AG Mason is rather sketchy - I'm speculating that they were making machine for Florence at the end, then got involved with Domestic, and when White took over Domestic, AG Mason switched to selling White made machines.
There are quite a few similaritys in style between the Florence and early White, but they also have enough differences - I sort of wonder if maybe AG Mason Mfg was actually making both of them and then White went to Mfg their own machines - and AG Mason became a reseller and not a Mfg . . . Sources all pretty much agree that the machines they sold later are White made machines - the confusing part is if AG Mason EVER actually made any machines, or if they were always a reseller . . .
I did verify this machine takes Boye Shuttle #25 - however it appears to take the same feet as White, though the throat plates are different. If this is the case, it will be the only machine that has the same spread between the needle bar and presser bar as the White made machine.
I'd love to see the underside of the early White to compare with this machine (and the next gen Whites also).