And for further info, Here is a picture of the two sizes of oil windows found on the L6.

The one of the left is the DW4064/A which is 7/8" O/D. I believe this is the one that is used on the M250, although the manual seems to misprint the part number for the oil window in the saddle.
The one of the right is the DW4062/A which is 1-1/2" O/D
Hope this helps
Graham