The chrome Morleys were made during the late 1970s, all the way up to the start of the 1980s... and then were replaced by a different series. The chrome ones are often known as "Tel-Ray" era pedals - because that's when Tel-Ray owned the Morley brand.
Those things are huge and have the widest sweep of any wah pedal you'll ever see - both in physical sweep (try moving it with your foot and you'll understand) and frequency range (from rumbling low to screaming high). The wah circuit is based around an LDR (also called a photoresistor) and a small light bulb; as you increase or decrease the amount of light the LDR receives, the resistance changes - the same concept you have with a pot in a traditional wah pedal, only it's done with light. They also run off an AC power cord - you can't use batteries with those beasts.
Modern Morley pedals are all LDR-based, too, but they now use LEDs instead of small light bulbs. They're also 9VDC powered and nowhere near as large as the old ones. The current reissue of the Power Wah is the only Morley I've used that comes close to sounding like the old ones.
The Power Wah Boost is the most sought after because of the Boost feature (and IIRC, it's the model that the late Cliff Burton used in his solos with Metallica). The others are also awesome, though.
PWBs, PWFs, and the Power Wahs with rotating speaker simulation (via the old oilcan method) go for over $100 on eBay, sometimes up to $200. How much did you get yours for, miacomet?
And they sound like no other wah ever made. You love em or hate em. 
