I enjoy trying to figure out stuff as much as anyone, but this is an issue for which there is no real answer. In this example, Parker built over 240,000 guns - so statistically it's not unusual to find that some were ordered without a safety and with long barrels. It's the same reason you'll see 8 billion red Camaro's and one in pink. Every so often someone walks in an orders something screwy.

I'm sure even back then there were 'catalog surfers' - guys who had a catalog and ordered strange combinations just because they could.