Jurgen
This looks like a post war engine, is it in a pre war chassis?, if post war then there was no plate to my knowledge between the engine & gearbox as the gearbox is supported with a rubber mount fitted to a support which straddles the chassis & underneath the gearbox!
Pre War cars (PVT) did use a plate of this type to resist engine torsion & offer some support, the gearbox also had support dependant upon which one was used, e.g. the ENV box had a tube through the gearbox which met the chassis rails with a rubber type mounting, you appear to have a Post War Engine with a Moss Box probably in a PVT chassis, that would account for the plate.
The gearbox bell housing flange is normally machined deeper to accommodate the plate thickness, if you remove the plate you will need to check the input shaft position to the rear of the crankshaft end as the input shaft has a bush into which the spigot on the crankshaft engages & runs, also you may need to make a spacer for the bell housing if the bell housing flange has been machined or the flange will not bolt flush with the rear of the engine flange so may crack the flange!