I'll maybe stick something through one of the lug holes and mark it then compare on the three drums, just to make sure they're the same wall thickness. Just trying to work out which of the screws I have are correct for the drums I have, before I start buying more.

Alright, the three shells, as far as I can tell from my shoddy measuring system, all measure around 4 mm thick, so I guess that means they should all have the same length of fixing screw.

I've taken a photo of the two main types of fixing that I took off the lugs on the drums. I'm assuming one of those is right, and one wrong. Going by the lengths on drummerfish's post, I think the top one (with the slot in the head) is yer medium length fixing, for 4 ply kits, and the other one hopefully is the 5/16 one that should go with my 3 ply kit?! Is that correct?

This should allow me to figure out how many fixings I actually need.

Richard, In the first photo the screw on bottom is the right one....In photo # 2 the screw on the left is the right....Mikey

I've cleaned most of my lugs in an ultrasonic cleaner and given them a polish with autosol. Some of them haven't any 'felts' and in some the felt felt slightly damp and came apart when touched. What is the purpose of the felts?

Felts in your lugs? As far as I know those aren't original in that era. They are added by people who want to stop the springs from rattling. I use cotton wool in mine. There are others here with much more experience than I have with the subtle changes over the eras, so maybe they can say if felts were ever put in at the factory. Anyway, this is a reversible retrofit so I wouldn't obsess over it. I tend to obsess over the pingy rattle sound which can come from the lugs if you don't pack them with something. Excited