... and talked to Kara. She said your gun is not an Agent but a Cobra, courier, or aircrewman, made in 1956, and that your barrel is an "AGENT" barrel made after 1962. It is not an original barrel or Agent gun.
I wouldn't dispute her answer, but might question it.
In 1956 the Cobra had the full-sized butt, not the shortened one that was introduced in 1953 on the Courier model. But it was only offered in .32 CNP (.32 S&W Long) and .22 L.R. with a 3" barrel. Making a Courier into an Agent would require much more then a barrel change. I would say possible but not probable. They only made 1,189 Aircrewmen revolvers on an exclusive Air Force contract. It had the full-sized Cobra frame and aluminum cylinder. Most were later destroyed, so simply finding one to make into an aftermarket Agent seems unlikely.
The 1955 date had to come from somewhere, and cocojo's single revolver wouldn't have been enough to cause it. What we really need to know is the shipping date on cocojo's gun, and Colt does have the shipping records.
I will propose an answer, with absolutely no evidence to support it. This is pure speculation.
After the Courier was discontinued (1956) and after the Agent was introduced (1962) an unknown number of unassembled Courier frames were found. Since the Courier was discontinued, these frames were assembled as Agents instead. Courier serial numbers run in the 26,275LW to 56,001LW range (more or less) and cocojo's Agent is 54,5xxLW.
If so, it wouldn't have been the first time that Colt had put old inventory to good use.
If someone has other thoughts I'm waiting to hear them...