The BBC Master is basically a BBC B on steroids; it has sideways RAM built in so that you can softload ROM images into it, which meant I could install certain "ROMs" into whatever machine I was using and have full word processing and DTP no matter which room I was in when at college (pretty handy, considering the number of rooms I got kicked out of). Note also the Electron Plus 1 style cartridge slots in the front. I'm note sure they were used much, as the BBC range was already much more advanced than the Electron and so not quite so reliant on third party upgrades, certainly not ones that plugged in to cartridge slots anyway.
This particular machine I bought for £20 from a car boot sale. The guy was going into his sales pitch, I was standing there nodding dumbly like I didn't know what a computer was, and I got him down from £25 because "that's all I have in my pocket". I'd actually drawn £40 out of the cash machine just in case. He thought he was getting a good deal because he knew he was talking bollocks when he spoke of all the software available for it - yeah, plenty of software, but a regular punter would have to go back in time a decade or so to find it in the shops. I knew I was getting a good deal for those sweet, sweet dual 40/80 double sided floppy drives - my BBC B only has a dodgy 40 track drive with no case, which won't handle all those floppies I manually converted from single to double-sided using a steel ruler and a Stanley knife then reformatted to 80 tracks. The casing to the drives is metal, more than strong enough to handle the monitor I've added to the setup since they're in a convenient side-by-side configuration rather than the up-down style I was used to at school/college.
Maybe the guy at the car boot got the last laugh - the keyboard's a bit shagged so I can't play Elite on it properly, and I specifically bought a "new" version of BBC disc Elite to get the Master version (second hand for a fiver at a Wakefield show). This is apparently a common problem, so beware; at least I have other machines I can use instead. There's also rumours of early models having problems with exploding batteries bursting their way out of the top of the case like a bad Alien parody.