I checked some older Colt catalogs I have,and the first year that I find the 5" bbl listed is 1963. Figured it would be earlier,as I have most years in the 50's,but not 60-62 years.
I have seen one in person,and posted something earlier this year about "who" a special run of 5 inchers were made for. It was the Venezulan Air Force,and around 15 years ago,SARCO or SAMCO in N.J. reimported a bunch. The finish was pretty well gone,as per typical of alloy framed pieces that spend most of life in a holster. Saw one at a show,about the same time,and I almost think it had plastic "Coltwood" stocks,which would put it 1951-54 or so. It had,IIRC,a square butt,like many of the 4" Cobras had. Pretty sure Venezulan "crest" was on sideplate or frame.
Often wondered to whom the Venezulan Air Force issued them? Pretty long bbl. for cockpit carry! but probably the Security Police,like I was in in the USAF.
Being "just a shooter",I find smaller framed Colts and Smiths with longer than 4" bbls. to be a little muzzle light and hard to hold,and thats why I got rid of a new Police target and a 22/32 S&W.
5" and 6" bbls were offered on the post war Police Positive Specials,but not for too long. Often wondered if Colt "found" some 5" unmarked bbls. and made the Venezulans "a deal"?? But would love to know when,why,and who carried them in their Air Force?