What year was my Greener GP 12 gauge shotgun made serial number is T12381

Check this fizya response

https://www.google.com/search?rlz=1C1AFAB_enUS452US453&q=greener+gp+serial+numbers&sa=X&ved=2ahUKEwjisM-rjfzgAhXgHzQIHV26ApsQ1QIoBnoECAYQBw&biw=1536&bih=72

have inherited an Iver Johnson Champion 12 gauge single shot

https://www.gunvaluesboard.com/iver-johnson-shotgun-1878555.html

iver johnson single barrel champion shotguns with a single letter suffix to the serial number were manufactured between 1920 and 1929. it seem that most of the 410 were used quite a bit and very few are in excellent condition. the original finish was an oil finished stock, browned barrel and case hardened frame.

CHAMPION MODEL 36--------------------------------1909-1922
Single barrel; rebounding center mounted hammer; top lever operated break open design; gauges: 12, 16, and 20; barrel length: 28, 30, and 32 inches (features barrel and lug forged in one piece) bored full chock only; American black Walnut but* stock and fore-end; automatic extractors; weight 6¾ pounds; overall length with 30 inch barrel 45 inches; case hardened receiver (nickel available as option), browned barrel; automatic ejectors extra cost option; .410 bore, 24 and 28 gauges offered in 1913; 24 gauge dropped 1928; all gauges except 12 dropped after 1941; name changed to Champion single Barrel in 1917.
CHAMPION SINGLE BARREL---------------------------1923-1978
This is the same gun as the earlier Model 36 champion except that the smaller gauges are now built on the same frame as the large gagues. See above picture for specifications. The exact year production stopped has not yet been determined. Some of these single barrel shotguns have been seen with Canadian marking, these were most likely shipped to Canada and then marked as there is no evidence that Iver Johnson's Arms & Cycle Works ever had a manufacturing factory in Canada

Your model 12 was made sometime in 1915 probably early first quarter as Winchester site states that 1914 ended with serial no 79765 for the model 12

Need to know more info. What gauge is it? What model is it?

Nicknamed, the Perfect Repeater at its introduction, this shotgun se tthe standard for pump action shotguns throughout its fifty one (51) year high rate manufacture and production life. From 1912 until the shotgun was first discontinued by Winchester in 1963, nearly two million Model 12 shotguns were manufactured and produced in various barrel lengths and grades. Initially the M12 was chambered strictly for 20 gauge, however 12 and 16 gauge versions came out in or around 1914, and the 28 gauge version of the Winchester Model 12 came out in 1934. A .410 version was never produced; instead, a stripped down variation of the Model 12, commonly known as the Model 42, was derived directly from scaled drawings of the model 12 and produced instead. The year of manufacture for serial number 787927 is 1939. hope this helps! im pretty sure that this is right. check it out on the web.

Go to nra.org and do a search.Or go to gunsamerica .com and get into a shotgun chat forum... hope this helps you.