Yes, it is entirely possible they were custom made. Casinos and repair shops had disk stamping machines for custom jobs (I almost bought a disk stamping machine when I was in the business).
As I mentioned in a previous post, your reel combinations only paid out around 50 something percent, which was way below what most machines normnally paid, so more than likely you had reels that were custom made for fomeone who wanted to rip their customers off.
Again, most Mills machines take the same, standard set of reels. Unless you are absolutely set on reproducing the same exact reel strips, you should be able to get your machine working by putting in another set of Mills reels from another machine as long as it had the same symbols. You don't have to worry about the order the symbols are in as that is taken care of by the holes punched in the disks. Doing so will probably be a lot cheaper than having new custom strips reproduced.