Let me know what you find. I've got a spare o2 regulator I could take apart, but it might not be the same.
I found one in the toy box. All the passages are tiny until the low pressure side, then it opens up. Upon entry into the regulator there is a mesh that looks like radiator grill. Then there is very small dia passage to the needle, then out to the low pressure side. The connection on this one is std thread, so I'm sure some time in the past it was used for some other gas.