Contact Pete Rimmer he may know the answer to your two Questions if not it may need more research from the Benelli Club
I recently purchased a 1972 Benelli 250ss that I've been checking over. I noticed it's missing part #3171 internal oil seal seen under a spring pressing on the oil drain plug. My engine is missing that part is it necessary? If so, who would sell it in the UK?
The guys on the Wards Yahoo group may be able to help. See the suppliers list at the top of the classics section. There's no specialists left in the UK that we know of but it is likely to be a standard seal and once identified you'll be able to pop down to your local bearing supplier and most likely they'll have it.