This carbine is indeed a Texan. Marlin referred (as far as I can tell) to the earlier and later models of straight grip 336 carbines as Texans.
The serial number indicates that it was made in 1966.
RC is an abbreviation for "Regular Carbine".
The saddle ring was introduced to the RC Texans in 1951 and was discontinued in 1972.
RC Texans had a round-ended Winchester-style lever, which changed to the square-end lever in 1972.
The 336-RC line included a curved pistol grip 20" carbine, the straight grip carbine, and the longer barreled half-magazine rifle with the pistol grip.
RC models were available in .30-30 and .35 Remington.
When the 336-RC designation changed to 336C, I could not locate. However I own a 336C with an AD prefix serial number manufactured in early 1968, so the changeover must have occurred between 1966 and 1968.
I owned a 1980(?) 336 straight grip carbine which was stamped 336T which had the square-end lever, which was labeled on the box as a Texan as well.
Anyway, while this does not answer your question of value, hopefully this information is useful or interesting to you.
As a guess, I would say that your carbine, in the condition that you state, would be worth at least as much as a new Marlin 336C rifle.
You don't encounter these much any more and people do want original JM Marlins, as they are better made than JM Marlins of the 80s and 90s, and later, and MUCH better made than the Remington manufactured ones of today.
But, I really am no authority on the value.
