[Amps] Different 3-500Z amplifiers

[Paul Baldock]

I have 2 amps with 3-500Zs. The 3-500Z's have Eimac labeling. The 
anode of these has vertical fins.

I just acquired another amp with 3-500Zs. These tubes have no 
lebeling and have an anode with  a horizontal disks.

Could somebody educate me on the operational difference if any of 
these 2 very different looking 3-500Z's?

Thanks

- Paul