From a factory log issued at the beginning of 1958, summarizing production through Dec. 31, 1957:
987 produced 1926-1929
987F produced 1929-1935
987E produced 1935-1937 "remarks: Monometallic balance, Elinvar hairspring starting with #4447301, year 1935"
987A produced 1937-1948
987S produced 1940-1948 "remarks: sweep sec. hand"
Individual quantities made were not recorded: "Quantity of 987A includes total for 987, 987F, 987E, and 987A as of Dec. 31, 1946."
The more detailed production logs, listing production by case style and year, make no distinction at all between sub-grades; all are lumped together as 987.
Installing a 987F or E into a 987 case is no problem, but the reverse is not always true. The jewel settings of the 987 make it thicker than later ones. Some later cases were made correspondingly shallower to accommodate the fractionally thinner F and E subgrades. So in some instances, if you put a 987 into a 987F case you'll hear a sickening crunch when you snap the back on. That's the balance pivot snapping off as the case back presses on the jewel............ Been there, done that. Ouch.