Hope I can explain this right. I have a 67 383 block I just pulled the frost plugs out of. The plugs were a little strange. 1 was in the block the regular way but not in very far. 3 were styled like flat round plates like washers with no hole in them. There is an inside rim in the frost plug hole in the block. These flat disks fit in till they hit this rim. A regular frost plug wont go in very far because of this inside lip. Anybody seen this before? There was some silicon in evidence.