It sounds very much like a Texan to me.
The squared lever was introduced in 1971, and the gold-appearing brass saddle ring was discontinued in that year, so both could be legitimately found on your 336. The 20" bbl is also correct.
There were probably not a lot of Texans made with both the squared lever AND a saddle ring since that would only have happened in 1971 only. Also, the portion of the 336 production given over to the Texan model was very likely not that large, either.
I believe over half of collectibility is in the perception of same, so it would be tough to say just how collectible it is. There might be a few dedicated Texan collectors out there, but I'm guessing their number to be fairly small. Well-used Texans aren't all that tough to find, so I'd leave the one you have as is if it were mine.
If I hadn't just bought two rifles and depleted my fun account, I'd send you a PM. SW