Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
Dont think it ever came as a seperate part
And yet CMS quote that part number as 4.8 x 14.5 not 15. Odd.
91256-657-920 is indeed the oil seal for a 500 four and it is 4.8X15X4mm. Should be easy enough to pick up at simply bearings for not very much money. Here you go:https://simplybearings.co.uk/shop/advanced_search_result.php?search_in_description=1&keywords=4.8X15X4&
Having mine powder coated Silver Ted.