Dave,
There were 2 types of spring listed for the Mark series machines - normal springs that were taper ground on the O.D., and heavy springs that were the same diameter for the full length. If you look at the parts book, it shows the taper spring fitted with the large diameter to the top. Now, the taper effectively makes these springs multi-rate, and the general rule is that multi-rate springs should be fitted so that the 'weak' end is static - this reduces the chance of the weak end becoming coil bound (often the cause of spring failure) under shock load. When I was involved with Club spares, I got a lot of enquiries about broken springs - my advice was to turn the springs upside-down to reduce the chance of them ever becoming coil bound.
Regards,
Eddie.