I suspect that the sleeves are sewn to the body at the back of the shoulder (at least that's what I got from the artwork at the time) leaving an opening at the front, as for at the elbow and wrist; well... it could be sewn, or closed with a hook-and-eye or lacing. I'm leaning toward sewn at the elbow and hook-and-eye at the wrist, but that's just me guessing. (a hook-and-eye hides well, was available and means that the sleeve can be opened at the wrist for sliding over the hand.)

Just my theories, feel free to ignore 'em ;)