I've been reading an excerpt from EYE WITNESS TO J. EDGAR
HOOVER'S FBI, by Richard C. Coffman. You can Google it if
interested.
The author served in the FBI from 1950 to 1980 (30 years).
At the time of his training (1950) He says they had a choice
of the Colt Official Police or the S&W Military & Police (later
designated model 10). Both with 4" barrels. He chose the O.P.
Page 291-292 He describes the issue holster in detail, but does
not name the maker. From his description, I believe it is the
Heiser model 459.
The OP had been around since 1927 and became FBI issue in
the mid 1930s. turnerriver has said Heiser's 457 was in their
1938 catalog "designed for the FBI". So apparantly by 1950
the "dog ear" was added, making it the 459.
Sorry if this minutia makes your head hurt. I know there are
at least a few others as interested in it as am I.