I guess you are talking about an M20. From what I've learnt. The "Loaf" bum pad was fitted to some M20's, but generally they were fitted with "Acumen" pillions. Firstly the long version as on my bike. This type had a problem as it was too close the the riders coxic bone and could cause pain over rough ground. So some were fitted with a soft protection bump at the front. see the three following pictures.
However the ultimate answer was to fit a shorter version Acumen. Like this one I have with perfect original Rexene cover (which could be for sale) Ron