The change from Army Special to Official Police was not an abrupt one. The most obvious change was in the contour of the topstrap, from one that is smooth and curved from side to side with the sight groove in the center (early style typical of the Army Special) to one that looks like three flat surfaces with the sight groove in the middle of the center flat (newer style typical of the Official Police).
During the transition period, you might see either frame style with either an Army Special barrel or an Official Police barrel, and the barrel marking would denote what it was.
Most Army Special barrels have "July 4, 1905" as the last patent date on the top, but some have "Oct. 5, 1926" as the last patent date, as do the Official Police barrels.
About the only other differences are that the Official Police had a checked trigger while the Army Special trigger was smooth and the Official Police also had a somewhat wider front sight
I have a late Army Special in the 542 thousand serial number range that has the newer style frame and an Army Special barrel with the 1926 patent date. Other than the "Army Special" on the barrel and the smooth trigger and front sight width, it appears identical to my 1929 and a 1931 vintage Official Police revolvers.
I have to add, the above information seems to hold true for the .38 Specials, but these revolvers in .32-20 are a different matter, with Army Specials being made in that caliber long after 1927.
