One thing I noticed when looking at spring tanks is most of the small units are plastic. I happened to find three of these all metal vintage 1970 ones. Not shown; I refoamed the center dampener and freshened the glue attaching the transformer cores/coils. Here is also a 1970 IC amplifier magazine clipping making reference to these tanks.

On a side note have you ever looked up the history and science behind the spring reverb?

Please take care of each other.

UPDATE: I did fond a Local source of magnets with the correct polar pattern for DIY spring reverbs. (SM Magnetics) 

Please take care of each other.