Here's a solution that has worked from me, when I couldn't find a transfill.
Order 1 or 2 of the part: # 2F347 from from Grainger. They are $2.29 a piece (Male SAE 7/16"-20 to Male JIC 7/16"-20 Straight Hydraulic Hose Adapter Item # 2F347). The fitting is rated to 7500 psi.
The fitting will allow you to do a transfill via the HP ports on two 1st stage scuba regulators attached to separate tanks. When I've done it, I had one 1st stage with 2 HP ports, then I did the transfill out of one port, and monitored tank pressures from the other port.
Be careful, and make sure you have a 2nd stage attached to each 1st stage so you can release the pressure. If you need O2 clean, I wouldn't do this method, use a clean O2 whip instead.
FYI: this transfill option is SLOW b/c the hp hose inner dimension is very small. I think it took me 10-15 minutes to transfer 35 cuft.
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