PART NO.  SPL.PAGE  DESCRIPTION                                                           
17H7734   16.K.3    Caliper -R/H -(front brakes, 1600 and 1600-MK-II Std.model)
17H7735   16.K.3    Caliper -L/H -(front brakes, 1600 and 1600-MK-II Std.model)

17H7679   16.K.3    Seal -fluid channel (2) -(caliper, front brakes, 1600 and 1600-MK-II
    Std.model) -- (part number from Moss US = 180-285)

This is the 17H7579 rubber seal ring located in between two halves of the disc brake caliper. The Workshop Manual says do not split the caliper for service. But it also talks about what to do if it does have to be split, and replacing this seal ring is part of putting it back together.

The actual rubber ring is much smaller than the picture, with bore size slightly more then 3/8" (10-mm). Unfortunately the replacement seal rings are not all the same size from different suppliers, and maybe even different from one supplier from one order to the next. I recently procured one from Moss Motors USA, and it appears to be different proportions and different material (or color) from the picture presented on their web site. So I have measured this one example:

ID = 0.412" -- OD = 0.600" -- Radial Width = 0.094" -- Thickness = 0.117"

We may get dimensions of more rings from other sources.

I will be asking for dimensions of the gland space in the caliper parts to determine an appropriate size for the rubber seal.