Colt might well have had unfinished frames on hand, but they were not numbered. The numbering was not done until the guns were fitted prior to final finish and assembly. Numbered but unfinished guns might have been in work, but certainly not that many, as batches were usually of 100 guns.
We have no way of knowing how many finished but unsold guns were on hand at the factory when the company decided to convert them. Serial numbers would be no help, since guns were shipped at random from stock, and many high number guns had undoubtedly been sold as percussion revolvers.
Incidentally, the book mentioned below does claim that fully finished Model 1849 revolvers were converted, including milling the frame for the rebated cylinders. Those would presumably be those with high serial numbers. This is borne out by close examination of the serial numbers. If you have a copy of that book, look on page 79, fig. 6-36; note that the barrel number is NOT from the same number set as the trigger guard and frame numbers - it is close, but not the same. That means that the guns #317605 and #318023 were numbered and completed, then converted with a new barrel which was numbered to match the old frame.
FWIW, my only gun of that type is no help, as it is a .38 RF Type 5, serial # 410x, identical to #1804 on Page 78, Fig 6-34, of the Breslin, Pirie, Price book. Like that one, mine is marked "36 CAL" on the trigger guard.
Jim