Dear friends,
I leaned a lot form this forums and I want to share my experience with you.

One of my customers had to make a small fridge with -50C low temperature for someone. First he bought small R404A (LBP) 1/2 Danfoss compressor and used it for this fridge.
By decreasing size of increasing length of capillary tube finally he couldn't be able to go to lower than -42C.

I suggested him use HBP R134a 1/2 Danfoss compressor and use it for low temperature with R404A.
he used it and he could reach to -51C with 1 stage (No cascade and no auto-cascade).

As you know for going to low temperatures we must have more compressed refrigerant at discharge of compressors (something happen that in 2 stage compressors). I think discharge valve of R134a compressor is tighter than R404A compressor and by this reason we can use R134a compressor (with R404A refrigerant) to reach to -51C.
(Both of compressors was 1/2 horse power and I know displacement of these 2 compressors were different and R134A compressor had more displacement but I think tighter diameter of discharge valve helped us to go to lower temperature)

Thanks.