Just for the record, I was once told Jaguar XJ6 front pads were a good substitute for Silver Shadow rear pads, with a little modification. So, I bought a set from Autozone and found the "little modification" entailed cutting the pads down in length and filing (elongating) the holes for the locating pins.
Needless to say my order for SY1 pads went in to Flying Spares the same day.
1981-87, 20,000 series Silver Spirit and Silver Spur parts: -Transmission: As for Silver Shadow. -Front Brake Pads: Ferodo FDB1284 complete with sensor hole. The old sensor can be easily transferred over. Mintex MGB633 w/o 5mm hole which can be drilled. (ClassicGarage.com at $9.96 a set of 4) See "General Information" for brand cross reference. PIC:
-Rear Brake Pads: Mintex MGB537, Ferodo FDB-1296-A at $68.79 a set of 4 from: Rock Auto -Parking Brake Pads: Mintex MGB563 or TRW-GDB563 (ClassicGarage.com at about $90 a set of 4)