Agent and Cobra are (were) two different guns.
The Colt D-frame (later designation) was the basic six-shot, swing-out cylinder double action Colt revolver. In the beginning there was the Pocket Positive, then the Police Positive (1905) and then the Police Positive Special in 1908. Latter had longer frame and cylinder to accommodate the .32-20 and .38 Special length cartridges. Offered in a variety of lengths from 1-1/4 through six inch barrels, including two inch. Most had four inch. This was offered right into the 80s and possibly the 90s.
In 1927, strictly as marketing ploy, they started calling the 2" revolver the Detective Special, because there was no difference but for the barrel mark. Offered in various .32 and .38 calibers, but made its name in .38 Special. Butt was square until 1933, after which round butt was standard. Also offered was a three-inch, in .38 Spl only. This was THE "snub nose .38" of movies and stories until advent of the S&W Chiefs Special. Oh, yeah, S&W offered the M&P in 2", but it was much larger and heavier--over 31 oz, to the DS at 21 oz.. Think Jack Webb in the old "Dragnet" TV series, with his Bucheimer cross draw holster and Detective Special.
In 1950, Colt offered the Cobra: Exact same thing as the Det Specl but in aluminum alloy frame. ZIP! Weight down to 15 oz. The Cobra, Aircrewman, Courier, and Agent revolvers were all numbered in same series, beginning with 1LW in 1950.
In 1962, Colt offered the Agent--Same as Cobra, with shortened butt frame. Now 14 ounces and a bit more compact. In 1966, all the D-frame Colts had frames shortened to a pretty standard length of metal and the different butt sizes were controlled with how much wood stuck below frame, from Agent (None) to Cobra (some.)
Pocketman, you write of "a Cobra with Agent markings." It is probably an Agent, because the marking is on left side of barrel. Someone may have put longer stock panels onto it-- Not an uncommon practice, because the alloy frame revolvers with .38/44 and later Plus P .38 loads, and small handles, are pretty squirmy in the hand.
An older model Agent in very nice shape might well be worth $250 today. I gave somewhat less about three years ago for mine, but it had the proper stocks on it and was first year production. I'd discount value of one with replacement handles, unless they are exactly correct. Lot of people put the old Tenex (10-X, see? Cute.) and later Pachmayr rubber stocks on ‘em.
Oh, yeah, Colt put out a detuned Agent in late 70s or early 80s--Most had a very rough, gritty trigger, dull black Parkerized finish, and a shrouded ejector rod, like the late Detective Specials and Police Positives.
Good luck--Hope you enjoy the alloy frame .38s as much as I have.
RR
PS--While the lightweight Colts will take the +P loads safely, they WILL loosen up after a while. I suggest practicing mostly with wadcutters or standard velocity loads.
PPS--I was writing this reply off line and came back to post it. Glad to see that Stephen and Shiro have issued prior confirmation of some of what I have written. Some of above is direct observation, and much is from The Book of Colt Firearms, by Southerland and Wilson.
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