[Spridgets] rev counter for dynamo or alternator

[Alan East-Jones]

whats the difference between a rev counter for a car fitted with a dynamo and
one designed for a car fitted with an alternator,?
 Assuming car is negative earth. (grounded)
In other words,  can I fit a '72 smiths electric rev counter (neg earth) into
a frogeye fitted with a neg earth dynamo ?
I thought I could,  then read a comment which suggested I couldn't.
Anyone really know ?
cheers
Alan