Surround timeline

[nov_1981]

I appologize if this subject has been covered in Ted's bass anatomy threads and I missed it..

Curious, I have a 72 4000 with a surround that does not have the depression or dip that allows
the strings to get lower without actually touching the surround.
Other surrounds from this period I've seen have the depression.

Can anyone give a time line on the style of surrounds?

Thanks

[BAD RONBO, KiLLeR DWaRfS]

greetings ! my MD had the ground down surround yet my MJ had the even surround.