I did some further "research" on these switches. I even bought one that said it's proudly made in the USA by Gama Electronics. Says it's rated at up to 200amps.
The Gama ($6.95)one came today and I compared my Chinese made ($2.95)cut-off switch side by side and the only differences I note are:
-Gama has a black knob with a USA flag in the center - woopie
-The white plastic insulator is a tiny bit thicker, TINY bit
-The crimp that sandwiches the insulator between the two cast 1/2's is tighter on the Gama cut-off that I have. Others may be the same or different.
The castings are identical and I would be very surprised if they're not actually made in the same factory (in Asia).
Structurally it seems that even without the battery lead attached, there is a lot of weight hanging off the terminal and the pont of leverage is where the round part of the terminal clamp attaches to the square part that the knob screws into. If you add an (unsupported) heavy lead battery terminal and cable, each jolt from our heavy sprung front suspensions would test the castings. If the battery cable is extra length, the extra length adds to even more unsupported weight.
It's no doubt that yours has failed. But a remedy might be to support the negative cable in such a way as to lessen the downforce.
On the TR3-4A, the negative cable is attached to the firewall, ABOVE the plane of the battery connection. So this actually helps lessen the pull against the casting of the battery switch as there is no movement between the battery box and the firewall.
But on TR250-6 models, the ground is on the block and pulls downward, hastening the demise of the switch. Add the repeated motion of the engine torquing and the suspension travel between the chassis and the body, and it's a little like bending a piece of metal back and forth until you break it. It become brittle, then breaks. In this case the casting fractures.
So a simple fix or patch might be to route the ground next to the battery support rod and add a couple of zip ties to help hold the ground cable steady at the battery side. This to transfer the flex between the cable (where it is zip-tied) and the engine block where it attaches.
Just my observations. I still think the switch is a good idea in any case. Hope this didn't put you to sleep.