Except for poor Swiss-like gilding and a couple features, this 7J undersprung movement with hidden stud, applied index and steel balance would be an excellent example of a private label P.S. Bartelett (PSB), circa 1859. This movement is similar to James Russell 19970 and 20093 except for the the French company symbol "Cie", to 20242, and to the 11J 21147 and 22177 movements except for the location of the signature. A comparison with 21147 is documented here.
The plates and location of all screw and arbor holes are identical between 20590 and 21147. The dials are interchangeable. The trains look standard Model 57 with 3-arm blunt-end escape wheel and enclosed English style pallet. The cantilevered potance on both look like the standard M57 part (the early part #215). The applied index and barrel bridge on both movements measure the same. The balance bridges measure the same but are not quite interchangeable (close though). The long pillar plate bridges and the oblong ratchet bridges measure the same. The winding arbors and guard cups look identical. A Waltham-like subassembly number is stamped on the pillar plates and under the top plates and balance bridges in the same manner as on standard Model 57 movements.
Some differences: The pillar plate of 20590 has the Waltham M57 peep holes with which to view the escapement in motion; Russell 21147 does not have peep holes. The regulators look identical except the tip on 21147 is a bit thicker. Although everything measures the same with my tools, something is out of tolerance because the steady pin for the cantilevered potance on 21147 is partially covered by the barrel bridge. The back of the dial on 20590 has the number 590 scratched on it; nothing on back of 21147. The full Russell number 20590 is not stamped on the under side of the top plate as on 21147. Russell 21147 has 11 jewels, but the top plate jewels are 2mm garnet jewels burnished into the plate like on William Ellery grade Model 57 movements circa 1871-1872, a decade after the hidden stud PSB! The mystery continues.