I took a look at a B78 this morning and I think I see what you mean. With the action closed, the breech block barely exposes a cut in the left side.
I think this is normal and yours has not been modified. The schematic shows a fairly large notch in the left side.
If this is for gas control as Eviltwin suggests, then I think the notch should not extend above the sidewall. This may have been a manufacturing error.
You are very fortunate to have a .243 as these are quite rare in B78's. If you feel it is unsafe, feel free to send it to me for proper disposal
Jerry