I apologize for nitpicking, but wouldn't a good explanation for a steel Agent be somebody who wanted a Detective Special, couldn't get one, but could get a Police Positive and an Agent barrel?
Also, I once heard that a number of Colt Agents got turned into Colt Vipers when Colt surplussed out some Viper barrels, because a Viper was worth more to collectors than an Agent was to shooters. That would also account for a supply of spare Agent barrels.
Although that would be a possible explanation, my steel "agent" probably wasn't a PP,
If swapped, to properly mimic an agent (steel though) with the polished barrel, the frame had to have come from another steel d frame with a polished bluing and short grip frame. I don't know Colts well enough to know which ones could be a possible donor, but the serial number on mine has a detective special prefix. I think the later PP's had the shortened grip frame too, so that would be a possibility for other steel "agents".
My glimmer of hope that it left the factory in its present condition is the date of manufacture of the frame. If I remember right, its right in there during the time in the 1980's that the Colt polishers were on strike, and the agent barrel is shrouded which also matches that time period.
So there is a chance that Colt just threw together all the extra polished agent barrels onto all the extra polished D frames, (which at that time were all short grip frames i believe) before they moved on to the parkerized production.
I'd have figured there would be something to confirm this if it were true, but I haven't run across anything yet. I don't know if Colt would even bother changing the serial number on a parts bin gun like that to match the series the barrel was marked as, but probably not.
The vipers would be a tricky one, and I wouldn't trust a viper that had a serial number outside the viper range for just that reason. You would probably be able to see a mismatch in the finish too. I don't think the Agent frame was polished as nicely as the Vipers were, and an aluminum anodized frame would be hard to rematch to the Viper barrel if someone were to attempt to refinish to match.
Any info or corrections would be most welcome, my knowledge of Colts is sadly lacking.