The Alamo Commemorative was part of a special run of model 30 (like a 336) Marlins, made for Oklahoma Tire and Supply Co. (OTASCO)
These guns were available in a centerfire using the standard 336 style action, with a birch stock, and the markings as found on your gun. A .22 based on the model 60 semiauto .22 was also available, and was called the 6081. The 81 designates the year it was made and the serial numbers on all the OTASCO rifles started with a special prefix. Your 3081 should be a OR prefix, while the matching model 6081 would have a OZ prefix.
These guns were first produced in 1978, and ran into the late 1980's. They generally have not gotten any real collectability, due to the less expensive wood that was ordered by OTASCO. They run about equal in value to other Marlin model 30a rifles.
Hope this helps.