Found this on google
I Have a deban catalogue infront of me and the listings are.
Mk1 collar will only work with mk1 locator.
Mk2 collar works with mk1, mk2 and mk3 locator
Mk3 collar will work with mk1 and mk3 locator.
and I just got a mark 3 collar that works with my mark 1 box to confirm the above but apparently a mark 1 collar wont work with a mark 3 box according too google anyway