dc4bs,
I ran into a problem just like yours. I wanted a DS4 w/ ATX50 to use for deco, because I'm going to be taking the IANTD Advanced Nitrox class which calls for using 50/50. However, when my LDS looked in the Aqua Lung catalog, he found that the "nitrox version" of the regs was not listed. So he called and this is what he was told... from the factory the regs can be used up to 40% with no modification. Once the regs have been used with non-filtered air though, they can (and I use the work "can" in the sense of what Apeks claims) only be used up to 23%. To use the regs from the factory with > 40% the O-rings need to be replaced with O2 compatible O-rings. To use the regs with > 40% after being used with non-filtered air, the regs need to be oxygen cleaned and have the O-rings replaced with O2 compatible O-rings.
If you look on the Apeks website (Apeks, not Sea-Quest) you'll find this, which I believe is what you are looking for...
Standard Apeks regulators can be professionally converted to oxygen service and oxygen compatibility kits are available to Apeks dealers together with instructions for O2 use.