It is the motors' stall torque rating, in ounce-inches.
The torque at zero RPM. This doesn't mean that the motors will develop these numbers while they are rotating at speed, all stepper motors drop off in torque as they begin to spin. What you will find is that those motors which have a lower inductance will exhibit less torque drop-off. That said, I've used the 381 oz.in motors on a couple of projects and I think a lot of others have too, for the mill you have they are a good match.
LongRat
www.fulloption.co.uk