any way to tell the date of an 8 3/4 rear?

Well, the 70 and later housing has the vent tube down on the drivers side axle tube, whereas the pre-70 style will be up on the hump of the center section somewhat!! This won't tell you what Suregrip unit is inside, it could be either! However, the 489 case was introduced in 69, and with it came the Auburn cone style SG, so a general rule of thumb is that if its a 489, it SHOULD have the cone style SG! Of course it could've been changed, and nowadays it's a much more common occurance than it was in the past!! Hope this helps some!! Geof