What is my seal called?

I have a 1970 service manual and they show a exploded view of the differential (terrible picture!) and it shows a pinion spacer but doesn't say it's a collapsible spacer (crush sleeve) like what's in a 489 case 8-3/4 unit. Looks more like a solid spacer like a 741 or 742 uses. But at the same time they don't show shims like 741's and 742's have?? With that in mind what I've typed is probably of little help other than to say that if you do it just like 71Scamper said it should be fine. And I agree with him to turn it just a little past where it was, not much, just about an 1/8" farther. That should seat and preload the pinion bearings just fine. Also make sure you use some red Loctite on the threads of the nut because it's under a lot of load pulling the whole weight of the car and I have seen them come loose. The Loctite will prevent it from coming loose.

BTW: it's called a YOKE (like used to harness an ox) , not a YOLK (that's a part of an egg)