Thanks, Michael.
After taking all the advice received into consideration, we've dedcided that the only acceptable way to err is on the side of caution. So Randy's going to pick up 4 cf of AC next time he's out to the factory in Colton -- then at his convenience we'll dump and refill the two cylinders ourselves.
It's just not worth it to take a chance on that copper & zinc combo when my kit can't test down to the level I'd need to to be sure that it's safe.