Were too. The B339s sent to the RAF weren't delivered with the specified engine and the airframe was the same initially but modified after delivery.
B339 was essentially an export version of the Brewster's (then) upcoming F2A-2. The production of the first 339s (Belgian order) started April '40. The B339s for Britain were produced starting from May '40. In August, the production was momentarily switched to
F2A-2s. January 21st, the first
F2A-3s are ordered and the delliveries started in July '41. More than a year after the production of the British order started.
Jukka Raunio also mentions it as "an alternation" of the F2A-2. F2A-3 also feautred additional fuel capacity which neither F2A-2 nor B339s had. All F2A-2s and B339s could carry 160gals like the B239 but the F2A-3s could carry 240gals.
Source: Francis Dean's America's Hundred Thousand, Pilots View Point II by Jukka Raunio
Like I've said, F2A-3 is a F2A-3 and B339 is a B339. No point what so ever to talk about them under the same designation, especially with a wrong one.