Crank Pulley: Dynamo vs Alternator

[A5H]

Trying to do something as simple as buying a fan belt has me scratching my head a bit. :?

Reading the VWH product page advises: "The belt relates to the bottom pulley that was originally fitted to dynamo models"

So my question is how do you tell the difference between a 'Dynamo' bottom pulley and an 'Alternator' bottom pulley when the history of my original engine (since been replaced) is patchy at best

 

[Doris]

don't know all the details but the correct size for dyno pulley is 905 x 9.5.

some belts are solid others have teeth. not sure which are which.

 

[A5H]

Yeah, it seems the dynamo belts are 9.5x905 and Alt belts 11.3x912, the dyno belts seem to be of the toothed variety.

What I can't tell is what the difference is on the bottom pulley, which is what dictates which belt to use, apparently.