JimmyB
Now retired - trains and fishing

Paul,I had exactly the same issue with an analog 'Rusty' (4 screws in line bottom plate.) Due to the difficulty in obtaining a new one I swapped out the centre gearbox / chassis section and bottom plate for an 'Otto' one (4 screws in a rectangle shape.) The original top plate fitted. Job done.
Regards Paul.
I noticed the similarities with my Otto, just need to find an Otto centre gearbox going cheap.