Well other stereos I have seen had a SW on the back.
The other possability is a production change. You could contact
jordan pier on U-tube. He is as good as they come with audio.
IIRC you said a Zenith worked OK. Probably has much less output
than a tetrad. The 2-G carts were only used on Zenith rigs, not
ever seen on add ons or other brands. So the phono amp can be
what ever the designers want.
73 Zeno
LFOD !