Not sure I understand you. The RMC seats on the conversions for the X15 were covered in standard RM moquette, they were not swapped for standard RM seat frames. The squabs are different as well having a deeper fitting piece to accommodate the deeper cushion. So in summary nothing on the RMC seat is standard RM.

My bus number (if any): RTL 960, RM 1585 and several RTs

The seat springs are the same. :)

My bus number (if any): RML2532

No they are not Roy. To accommodate the larger cushion they are much deeper and many are made of fibre glass.
If you mean in the seats the horse hair used came only from country area bred horses.... :)

My bus number (if any): RTL 960, RM 1585 and several RTs