Been gone all day, so I am late to the party. However I concur 100% with kkkriverrats. The first production 1919A4s did not come about until April of 1941, from Saginaw Steering Gear. Both RIA and Buffalo Arms got into production after that.
Steve points out that the current repros are advertised as "Colt" repros. That is not inaccurate, the only caveat being that Colt was not the only one producing guns with the slotted jacket. Westinghouse, Marlin-Rockwell as well as Springfield Armory would have made them. Here is where the confusion lies. The slotted shroud first appeared on the 1918 Aircraft gun, and shorter versions on the 1919 Tank gun and its 1919A2 sibling. The A4 went to paper about 1936, so the slotted shroud was still the standard. All A4s- with perhaps rare exceptions- were converted as stated. Sure, a few new receivers may have been built during development, but no "production" of any kind took place until 1941. We don't know how many "conversion" A4s existed before Saginaw got in gear (sorry!). It may have been hundreds, it may have been thousands. We do have information, though, that the conversion program at RIA continued right through WWII.
Oh, as to Steve's question about the company making slotted jackets making some with holes... Well, they have made A6 jackets. I think there is just no demand for new made A4 jackets with holes, since there are plenty available surplus in good condition, and for cheaper than they can be made.
