I picked up an RF adaptor to replace my Famicom one that died, and I had been using for my PC Engine. Anyway, it was just a cheap, fancily packaged HK one from Retro-Bit. Let's take a look at the back of the box. (I don't have good pictures of the cable itself...)

Notice how it contradicts itself, it says it goes to Channel 13 on the left side, but 3 or 4 on the paragraph on the right! Also, there's no way you could "seamlessly" switch from TV to game unless you have some kind of dual RF adaptor. It comes with a standard RF to coax cable, and a box that basically just works as an adaptor for a Sega Genesis Model 2. There's no other coax port on the box itself unlike the NES-101 gray box model.
Le sigh... either way it works alright with my PC Engine. Not the best picture, but I've seen much worse.