Parts availability during the 1970s might be a good clue.

The history of the BT-55V body tube and BNC-55AC nose cone:

1967 - 1971: The 16.35" long BT-55V was listed in the Estes catalog

1967 - 1974: BNC-55AC listed in Estes catalog

1969 - 1981: BT-55V kit Avenger released

1970 - 1977: BT-55V kit ARCAS released

1970 - 1983: BT-55V kit Cherokee-D released

1971 - 1982: BT-55V kit Bandit released

1974: The BT-55V is listed in the Estes Custom Parts Catalog

1975 - present: PNC-55AC listed in Estes catalog, in one way or another

1975: second 18" BT-55 kit released A-20 Demon. This marks the beginning of the 3FNC and 4FNC BT-55 kits. The first was the Interceptor in 1971, but I consider that to be an exception.

1980 - 1983: Last of the BT-55V kits released - Skybolt

1983 sort of marks the end of the Estes "Golden Age" with the demise of the BT-55V.

I wonder Estes switched some of the other BT-55V kits, the ARCAS, Avenger, etc. from BT-55V to BT-55. I'm guess that some did, but I'll dig out an unbuilt ARCAS kit from those days and check. Another speculation is that the switch was transitional, but by 1975 it was complete (full length BT-55 kits make an appearance, and the BNC-55AC nose cone is gone).
-W

__________________
-Wolfram v. Kiparski
NAR 28643 - TRA 15520
MTMA Section #606 President