No o-ring. As previously mentioned, it will interfere with the operation of the vacuum advance. But now that you have the distributor out, take the top half apart and remove the point plate. You might be surprised by the amount of junk, crud, rust. Brush mineral spirits in until its as clean as you can get it. Once nice and clean, get a little light oil on the advance weights and make sure they are free to move in and out. Check the pins holding the springs. Make sure they are tight and that the springs are both in place. Little grease on the pivots where the springs are attached. I use whatever I have around, usually red or blue wheel bearing grease. Put it back together, reset the points and you have done as much servicing of the mechanical advance short of taking the entire assembly apart.
