The holster you have there is called an Audley. It was an expensive holster, and at the time it was made, it may have cost more than the Ruby pistol you have in it. In good condition, it might still sell for more.
Because of the nature of its retention system, Audley had to make specific models of holsters for specific models of pistols, and in the case of revolvers, for specific barrel lengths as well. Because the Colt 1903 was the most popular US 32 automatic of that time, there are more Audleys for it than for other pistols of that size, so I would guess that is what yours was originally intended for.
Audley holsters that size were also made for Savage automatics (both 32 and 380), and the Remington Model 51, as well as for a wide variety of other pistols. None were made for Rubies; Rubies varied too widely in dimensions for one thing. For another (and meaning no offense), it would have been like making a high-end Safariland rig for a Hi-Point 9mm pistol - a person who bought one would be unlikely to buy the other.
There was an excellent article about Audley holsters published a long time ago. I think that it was by Donald Simmons, and that is was in the first or second "Gun Collectors Digest", but either is long out of print.
One last note - the main period of Audley holster production seems to have been after World War 1, in the 1920's and 30's. Very late Audley's were made under the name "Folsom Audley" and were often for police-type revolvers. Their quality is noticeably inferior to the earlier Audleys. None have been made since the 1950's, I believe, and the earlier ones, at least, are heavily collected in their own right. You can see the prices asked or realized on Ebay or the major gun-auction websites.
PS - the Audley retention device was patented, so they are the only maker of it of which I am aware. There may have been copies after the patent expired (I believe it was issued in 1914), but the Audley was not an easy holster to make, certainly not for a low price.