Mainshaft oilseal replacement

I need to replace the oil seal on my gearbox mainshaft. Does anybody have a generic replacement part number (Norton part 040132)?

Also, I assume that the lip of the seal should go around the countershaft sprocket spacer and not just in front of it, right? Does that mean I have to "tuck" the seal rubber in, or should I put the spacer in after the seal is in?