SAY WHAT!!? A *MYSTERY* Crankcase Cap That's Neither Metric, nor Imperial!

The other day the former owner of EZ-RIDER dropped by. He looked the bike over, gave me some encouraging words and then handed me a replacement Crankcase Cap and O-Ring that he had purchased from HONDA to replace the damaged one on the motorcycle. Why a new cap? Because he was using it to replenish the OIL in the motor! But what about the filler cap / dipstick that the 1997 Honda Magna VT250CR has, you may ask? Well, I'm glad you asked.... The story goes like this: EZ-RIDER is heavily modified. One of the niftier modifications is an "old school" exhaust system that brings two tailpipes together on the right side of the motorcycle. Undeniably cool, this beautification came with one downside: The aftermarket exhaust completely blocks access to the oil filler cap / dipstick. There's NO WAY to get it out, save removing the exhaust! So, the owner started to remove the crankcase cap instead, whenever he felt it necessary to top up the oil in the engine (hmmm....that sounds ominous...why would it need to be topped up?). Over time, he said, the "corners got rounded" on the crankcase cap (strange, that) to the point where he said he could not open it any more, so he ordered a replacement from Honda. Then, he put the bike aside and replaced it with a Harley Davidson sportster. Weeks later, the replacement cap and o-ring arrived, so he collected them and put them on a shelf. Now, they have ended up with me. Pleased to no end, I happily accepted the new right crankcase cap and o-ring, but they came with a mystery - it seems that the right side crankcase cap is 16.5MM in size! This is VERY WEIRD. I've never seen a fitting that accepted *neither* metric nor imperial sockets! Have you? Honda Crankcase Cap Part #: 11332-MZ5-000 Honda Crankcase Cap O-Ring Part #: 91301-ML7-003