Odd Brtish ID models

[grwebster]

Most wartime British wood ID models did not have a hole through the CG or any obvious way to hang them like the US series. I suppose it was left to the end user to develop a way to use them.
Here are some with an unusual rectangular socket in the bottom of the fuselage.


I have never found any explanation for this but it must have been for use on some type of training devise. Anyone know?

I have a few others like this in wood and even a post war metal Mig with the same unusual rectangular hole.