It is a ridiculous system and I once witnessed its absurdity.
Several years ago, before I had my CHP, I was registering a recent pawn shop find with OPD. There was another person there who was also registering a gun, but he had it in his possession as he had bought it face-to-face. He had a purchase certificate issued by the Douglas County Sheriff.
When he tried to register the handgun (a third generation S&W) he was told it was a no go and they could not give it back to him because he couldn't legally register it. The problem was that he had a conviction for disorderly conduct, related to the discharge of a firearm in the city limits. This didn't disqualify him from getting the certificate that the Sheriff issued, but the city said it was a firearms offense so he couldn't be issued a registration card.
He left without his newly purchased gun, which the Sheriff green lighted, but OPD confiscated. It was a mess.
I almost offered to buy it from him on the spot. I figured, it was legal for him to own, but the city wouldn't let him possess it, so he should be able to sell it to someone who could legally own it. I could then apply to register it and take possession. I'm sorry to say I didn't do it. I doubted the officer working the window would have understood the situation and it would have caused me more hassles than worthwhile.