The "Trapper" or "Claw" style first base mitts were produced by Rawlings and featured a "three-finger" design with intricate lacing running along both sides of the "middle" finger. I'm not real big on first base mitts, but believe that this design was introduced in 1941. The "Trapper" web was a precursor to the "Trap-Eze" fielder's gloves that Rawlings introduced in 1960.
The T-90 model had a long run, from 1942 up through the late 1950's. From what I can gather, these mitts bore the endorsement of Tony Lupien between the years of 1943 and 1957. Lupien's career itself didn't last as long. Yet Rawlings sold gloves with his endorsement for nearly a decade after he was out of the majors!
Beginning in 1940, he played only four full seasons and parts of two others, missing a couple of seasons due to World War II. After his Major League career ended, Lupien moved on to the Pacific Coast League, then became the coach at Dartmouth College for over twenty years. He also had a pioneering role relating to player's rights following the war. His rather interesting biography can be read here:
Lupien Biography
Back to the glove: There may be slight design variations to the glove that can help you nail down a more precise date. One example would be variations in the cloth Rawlings patch on the glove- if it hasn't been lost to time as many are.
Another possible design detail might be the use of a "web controller" band across the back of the webbing. Again, first base mitts are not my most familiar area, but I believe this leather band was added to these mitts in 1950, as a reaction to a rule change regarding the size of gloves and the their webs. The band was intended to prevent to web from being expanded to a bigger than legal size.
I know that there are some members here with more expertise in first base mitts. Maybe one of them will have some better info for you as far as the exact year of the mitt.