Gentlemen, I may be able to add some info. This afternoon I stripped a Rover 45 with badly cracked front and rear rails (the front had broken through). Everything appeared to be factory original. The mower was fitted with a Briggs 91202 manufactured in 1993.

Here is a photo of the clutch, once removed. Obviously it is missing a nut, which doesn't help, but the counterweight is present.

[Linked Image]

So I checked my Rover daily driver, also a 91202 from 1993 and the counterweight is present. Thankfully the chassis cracks are not wink