The cabinet is by S.G. Brown and I think dates from around 1926-8 - this is a fairly similar example with a slightly different fret design:
https://www.radiomuseum.org/r/brown_duplex_v12.html
My guess would be the loudspeaker fitted isn't original, but dates from a few years later. Moving-iron drive units such as the one shown in the link above had almost vanished by the early '30s, supplanted by moving coil designs capable of much better performance and essentially very similar to most speakers to this day: so many perfectly good cabinets were improved with new and better drivers in the early '30s. I couldn't say who made the present unit, but someone here will probably know. PM in its model designation will stand for Permanent Magnet, as distinct from the energised magnet speakers fitted to many mains radios of the day.
Paul