Hi moeburn
I do not know whether TWOS has any influence or not on this?
I have played previously with my own mod set up and have been able to use it when upgraded to the VIIB.
Historically I do not think it was used on VIIB but on the type IX.
In your charts you will find the upgrade chart.
It is quite possible that when you are offered the upgrade from the VIIA to VIIB you will be able to buy the bigger deck gun.
In the meantime I can recommend the Critical hits deck gun mod.
Peter