Most 3+1 compensating euphoniums have a swinging cover that can hold down the 4th valve. I don't know what it's for; I just assume it is there to protect the valve when not in use. But is it ever used while playing? With the 4th valve depressed, the instument is effectively a small F tuba. But with only 3 valves, some slide pulling would help. So the valve lock could hold the valve down, freeing up the left hand to pull slides, finally allowing an in-tune low B.
But that seems awfully fussy and complicated.