Many have three mounting holes top and bottom which accommodates either single or two gang mounting. If it doesn't have the center hole then it would probably require the double deduction.
The wording in 2017 NEC 314.16(B)(4) is "A device or utilization equipment wider than a single 50 mm (2 in.) device box as described in Table 314.16(A) shall have double volume allowances provided for each gang
required for mounting."
I agree now that if you find a NEMA 14-50R that says it can be mounted in a single gang box, even though it is wider than 2", then it only requires one double allowance. Some of them say not to mount in a single gang box, in which case I would say they require two double allowances.
So that is one way to use a 35 in^3 double gang box, find a 14-50R that says it's OK to mount in a single gang box. It obviously still wouldn't fit in a single gang box, but I guess the labeling means you could use a single gang mud ring on a 4" square box.
Cheers, Wayne