Here is a little info I found on Contender barrels.
Type I
Term used to describe the very earliest of production Contender barrels (early 1967 to late 1967). These barrel are easily spotted by their "one line" company address and caliber designations. All Type I barrels are octagonal, have flat bottomed lugs, allen screw bolt retention and short style rear sights. To date, only six calibers are known to exist in this barrel type. They are 22LR, 22WRM, 22 Jet, 22 Hornet, 38 Special, and 357 Magnum. Type I barrels are known to exist in both 10 and 8 3/4 inch barrel lengths.
Type II
Designation given to the second style of octagonal Contender barrels (late 1967 to 1972). These barrels may be easily recognized by their two line address and caliber designations. Of further importance is the punctuation used between the "N" and the "H" used to abbreviate the words New Hampshire; both are followed by a period. (example - Rochester, N. H.). Type II barrels are also recognized by their flat bottomed lugs, allen screw bolt retention and short rear sights. Type II barrels exist in three lengths, 10 inch, 8 3/4 inch and 6 inch. These may also be found as "scope model" variations (w/o front sights). The largest selection of calibers exist within the Type II classification including all of the original 17 caliber offerings.
Type III
Third generation of octagon Contender barrels (1972-1973) possessing most of the same features as the Type II barrels, namely the flat bottomed lug, allen screw bolt retention and short rear sights, but differ ever so slightly having only one period following the abbreviation for New Hampshire (example N H.) Keep in mind, that due to the fact that rear sights are interchangeable, all barrel types may be found with incorrect styles of rear sights installed.
Type IV
The forth generation of octagon Contender barrels (1973 to 1977) look almost identical to Type III barrels. Address and caliber markings are identical, (N H. abbreviation) with both possessing the flat bottomed lug. Careful examination of the underside of the lug however will reveal the absence of the allen screw used to retain the bolt. All Type IV barrels utilize the new "roll pin" method of bolt retention. Long rear sights were used on all Type IV barrels.
Type V
Type V barrels represent pretty much the final stage in octagon barrel development (1977 to 1980's). The most readily noticeable change made to these barrels is the addition of material to the rear of the barrel lug just under the bolt which causes a step in the lug midway through length. Type V barrels will usually sport the second or third generation rear sights (long or improved style), but keep in mind that these sights are interchangeable and may not be original to the barrel. Writing styles may differ on Type V barrels as several different variations were used prior to the discontinuance of octagon barrels from the catalog. Later production Type V octagon barrels were perhaps some of the finest looking barrels ever made by Thompson / Center with their "super" high gloss blued finish and crisp barrel markings
