Found this on
http://www.roverholic.com/rover-v8-engine-number-database-86.html
engine no. car CR hp
42500001A Rover P6 3500 3528 10.5:1 164 Auto SU HS6 1967-76
42700001A Rover P6 3500 3528 8.5:1 Auto SU HS6 1967-76
43000001A Rover P6 3500S 3528 10.5:1 164 Auto SU HS6 1967-76
43200001A Rover P6 3500S 3528 8.5:1 Auto SU HS6 1967-76
45100001A Rover P6 3500 3528 10.5:1 164 Auto SU HIF6 1967-76
45100001D Rover P6 3500 3528 9.25:1 143 Auto SU HIF6 1967-76
45300001A Rover P6 3500 3528 8.5:1 Auto SU HIF6 1967-76
45500001A Rover P6 3500 3528 10.5:1 164 Auto SU HIF6 1967-76
46600001A Rover P6 3500S USA 3528 Auto SU HS6 1967-76
48100001A Rover P6 3500S 3528 10.5:1 164 Manual 4sp SU HIF6 1967-76
48100001C Rover P6 3500S 3528 10.5:1 164 Manual 4sp SU HIF6 1967-76
48100001D Rover P6 3500S 3528 9.25:1 143 Manual 4sp SU HIF6 1967-76
48500001A Rover P6 3500S 3528 10.5:1 164 Manual 4sp SU HIF6 1967-76
Whether you can use that extra power or not with super unleaded or octane booster though i really dont know!
Rich.