Bruce, An educated guess... I think because of content, that they just made them up utilizing techniques from the system. They start (White-orange) somewhat linear, then later belts blend hard-soft, circular-linear. Dynamic kicks mixed with fast hand combos (Chin na- Paqua) soft circle blocks, open hand strikes, mantis blocks. Grabs-throws combined with kicks and knee strikes. Slow Kihap - dynamic tension moves mixed in. Fun to do, great to watch.
If there is something that came from monks this might be part of it (In hyuk suh's hyungs too) Lets not discount the Kung fu influence. There were evidently quite a few Kung Fu schools in the late 50's and 60's. I studied 6 months with a 64 year old Hapkido KJN a few years back. He was from GM jee han Jea but also studied kung fu and judo extensivly in the 60's. He did not speak much english but I gathered it was commen (Kung Fu).