The actual part number is 201-274-03-15, and that's for my 1991 300d.

No, he did not have to take the transmission out, and he didn't even have take the tail housing out.

On the MB transmission diagram which you can have emailed to you from your MB dealership, just from memory, I think it's part #145.

I bought another part just in case, 201-274-00-15 which is a plastic gear for the (?) rear pump. My mechanic turned the pump and felt the gear on there was fine, so we didn't use it. It would have been too much for him to take other parts off to get to the pump. He didn't hear any loose plastic parts rumbling around in there, so he passed on taking that area off. If that pump DOES have a problem in there, I heard it will not work properly, and cause the plastic part we replaced to break again. So if you can check this pump area for loose debris or broken gear of its own, you'll be in a much safer position.