The vented yoke is used on the th-475 and is a strictly bolt on deal. The "vent" is where the bolt passes through the yoke to the trans tailshaft. The vented yoke, like was stated, is sealed with an "O" ring and is machined internally about 1" deep. The "o" ring seal can only be used with the vented yoke in the stationary position.
A special yoke is required to use a slip yoke on a th-475.
Although I don't know of anyone who has done this, but I'm convinced that the th-475 vented yoke can be used as a slip yoke on the th-475 trans if the "vent" is sealed and the driveshaft if the proper length. The only reason I would see to do this is if you already have the vented yoke. You will need a different driveshaft anyways if you are replacing a Powerglide or a th-350..
Driveline shops are the ones that sell the "special" yoke, so it is not in their best interest to reuse the th-475 yoke.