Mossie and Il-2M work exactly the same.
In the case of the A-20G it starts with the bombs in the bomb bay selected (ordanance 1) and hitting Backspace switches to the bombs under the wings (ordanance 2).
In the case of the Mosquito you start with machine guns and cannons selected, hitting Backspace selects the bombs in the bomb bay (ordanance 1) and hitting Backspace a second time selects the bombs under the wings (ordanance 2).
In the case of the Il-2M you start with machine guns and cannons selected, hitting Backspace selects the bombs under the wings(ordanance 1) and hitting Backspace a second time selects the bombs in the bomb bays (ordanance 2).
So, the A-20G and Mosquito go bomb bay first, then wings. (ordanance position 1 is the bomb bay and ordanace position 2 is the wings)
The Il-2M goes wings first, then bomb bays. (ordanance position 1 is the wings and ordanance position 2 are the bomb bays)
It'd be nice if they all worked the same, but ah well.
You just have to remember what you have selected.