manual trans plug size

I have a 1991 silverado with 5 speed manual transmission. I need to check the transmission fluid. I found the plugs but they are not standard plugs. Does anyone know the size and is it metric or SAE. I measured with verneer calipers and it measures 16.88 mm ,flat to flat, of what appears to be over sized allen wrench. Is this a special tool and where can I find it or does someone know how I can get this plug out without damaging it. Thanks for any help you can give me.

I have a 1991 silverado with 5 speed manual transmission. I need to check the transmission fluid. I found the plugs but they are not standard plugs. Does anyone know the size and is it metric or SAE. I measured with verneer calipers and it measures 16.88 mm ,flat to flat, of what appears to be over sized allen wrench. Is this a special tool and where can I find it or does someone know how I can get this plug out without damaging it. Thanks for any help you can give me.

use a bolt head fits great...much cheaper ..vise grip on the stud threads...

Thanks thats a great idea and a lot cheaper.