btw, it looks a little "shady" in that it is missing a cover plate screw, mis-matched color screw, and I think (but others will chime in to state correctly) a 6-50 requires a 2-gang box?
A single-gang box will not permit a 50-amp receptacle for 14-50 with Romex but is sufficient for other configurations.
314.16(B) dictates box fill requirements. A single-gang box (shallow, 1 3/4") allows 18 cu in., and a deep single-gang allows 24 cu in.
Each conductor has a specific volume requirement - #6 needs 5 cu in., #8 requires 3 cu in., #10 requires 2.5 cu in. - includes grounding conductor
Receptacles count for 2 conductors each at the maximum conductor size attached.
Cable clamps for wire entry count as one if the clamp is inside the box (vs. outside, which doesn't count).
Loops in the conductors to permit pulling the receptacle out can count as another one, depending on how much wire was left.
For a 14-50 receptacle with 50A circuit, NM/Romex (#6 required):
3 x #6 + 1 x #10 = 17.5 cu in conductor volume req (2 hot + neutral + ground)
2 x #6 = 10.0 cu in receptacle volume req
27.5 cu in required (single gang doesn't work)
For a 14-50 receptacle with 50A circuit, wire-in-conduit (#8):
3 x #8 + 1 x #10 = 11.5 cu in conductor (2 hot + neutral + ground)
2 x #8 = 6 cu in receptacle volume
17.5 cu in volume, shallow single gang works provided no clamps required
For a 6-50 receptacle with 50A circuit, NM/Romex (#6 required):
2 x #6 + 1 x #10 = 12.5 cu in conductor (2 hot + ground)
2 x #6 = 10.0 cu in receptacle volume req
22.5 cu in required (deep single gang only will work, provided that no clamps inside the box are required)
6-50 will obviously work with 50A circuit, #8, one less wire.
(and yes, please get good cover screws on that cover! That single screw is a countersunk cover screw, the 6-50 and 14-50 requires pan-head)