The Frontier model was made available in the 1911 cartridge while E&M operated Great Western. Original examples should have a serial that correlates to post-1960 assembly, probably no earlier than GW20500.
This is the rarest factory chambering. There are only a few .45 ACPs residing with advanced collectors of Great Westerns. My statistical sense is that no more than a couple dozen were built.
Rollmark: .45 ACP
Models / barrels: Frontiers, Fast Draws, Kits (liquidation series).