How can I tell what year lower unit I have

[200090tlry]

Ok I got to thinking maybe my 2000 90tlry has different year lower unit? I lookup part number 71 on say 1993 90 horsepower and my year does not show this oring. What does the oring do below shift shaft housing small tiny thing. Driveshaft seals are same number so that rules that out. My shaft has the little indent for something to go there.

[boats.net]

Number 70 is the oring93210-07003-00
, number 71 is a circlip to keep shift shaft from coming out of gearcase, number 71 part number is 93430-08006-00 which is missing from parts diagram.

[200090tlry]

Yes I wondered why my lower is missing this part

Been trying to diagnose why lower unit vac test is leaking little bit. Ive gone through whole thing. Now im thinking hey maybe my 90tlry 2000 year has older lower unit installed as this was purchased from Florida motor rebuilder. The part number on like 93 90hp (71) looks like small rubber oring.