What’s up with the “Z”

Because the 2035 was Magne-Traction and the 2025 was not, I'm guessing the Z is for Zinc (non-magnetic)?  Or, it could have a more aggressive thread.  The parallel plates on Magne-Traction locos are aluminum, and it's easy to strip out a hole if you over-tighten a screw.

Some people love Magne-Traction, but I'll take the black steel parallel plate motors over the aluminum ones any time!!

I don't think it is magnetraction related specifically as the retainers are making frame contact around them; maybe something to do with the aluminum and galvanic corrosion?..oh, maybe it could be accelerated by magnetraction?

A chase cut or taper in the threads (self tapper?) 

It might not even be a Lionel specific hardware standard.

The “Z” refers to the type of self tapping threads. The type “Z” threads look something like a wood screw. Another common self tapping screw Lionel used was the type “F”, which looks like a machine screw with a taper and grooves in the threads that look a little like a tap.  Many other types were used. Machinery Handbook illustrates some of them. 

Thanks David!  So it IS a more aggressive thread, probably because it's going into softer aluminum, vs. steel.  I learned the hard way not to overdo it when working on Magne-Traction steam motors.

Yes, either softer or very thin material is where the type “Z” would be used. 

@David Johnston Thanks for the detailed information. Attached is a picture of the screw in question. Now comes the challenge on finding one, or at least a close second.

Nyerges Bolt  & Screw List now operated by @Len Carparelli is where I would first contact for that screw.