FN made changes throughout production. The "T" series is considered the high point of workmanship and command a premium - that would be early to late 60's. The craftsmanship, the finish, the fitting was at its highest point.
The "C" series is probably the second level as far as craftsmanship and design is concerned. Many people will argue that the C series is just as good as the T series, but the "T" just has a cache' that the market puts a premium on.
The earlier guns (prior to 62) have the internal extractor and poor sights, but are still very collectible. The internal extractor is slightly problematic - they break. They are more valued as collectibles than as shooters.
The later guns, after the "C" (the Mark II) don't have quite the finish and workmanship of the T and C series guns - production became more automated and less dependent on hand finishing.
All Hi Powers, even the latest ones, are superb firearms, but the T and to a slightly lesser extent, the C, were the high water mark of Hi Powers.
If your C is in good condition then it's one of the best firearms ever manufactured. Shoot it. Enjoy it.
