Bendover, they were not iterchangeable, the E wing had .50s inboard and 20mm cannon outboard (always). The C wing had (usually) 2 20mm inboard and the 4 .303s (in their standard positions).
The 20mm "could" be set outboard on the C wing, in the usually empty spot, but I was wondering if they ever used it in that position on the IXC.
Regards.