[Boatanchors] Whence the 3-500Z?

[shoppa_boatanchors at trailing-edge.com]

When did the 3-500Z hit the market? My '68 ARRL handbook has
construction articles for the 3-400Z and 3-1000Z but I don't see
the 3-500Z in it.

Is the 3-500Z an "improved" higher-dissipation 3-400Z or is there
more difference other than better dissipation?

I believe that soon I will find my 70's era handbooks but I haven't
seen them yet :-).

Tim.