273, could it be a commando and CAM question?

just wondering due to 727 on a 273 -- you ever crawed under the motor and looked at
the engine number stamping above and left of starter under exhaust manifold. Should be some numbers and then a 273, 318, 340, 0r 360. Curious if someone put a bigger motor in there for you - they look pretty much the same from the outside. Would be sweet if you really had a 340!!!