Back when I was building my A5 deployment kit, I sourced the correct 50SBC-ARD for my S&B 3-12 but it didn't come with the shipping tube. I went down a rabbit hole of research and found that the original tubes were Rose Plastics Twistpack Model DP 65 060. These were 65mm in diameter, adjustable in length from 2.36" - 3.15", and molded in black polyethylene. I never pursued ordering direct as they had min. order quantities of 1000 units.
If you ever end up needing just the packaging tube to complete your deployment kit, I was able to find one from another military issue ARD for the XM107 Mark 4 optic, the Tenebraex 50LCL-ARD. These seem to come up more regularly than the SSDS ARDs on auction sites and shipped originally with the same black tubes.