They are very nice locks mercury.

Chubb seemed to do that a lot- making their locks 'modular' single or double sided by changing covers, lever-sets and curtains etc. I've seen it on more recent detectors but didn't know they did it on earlier locks as well.

Don't be put off by wear or corrosion as it'd be hard to find any of that age in immaculate condition anyway. I'd be tempted to leave them as they are, as everyone has different ideas about how to clean and how far to take it. If you intend to keep them and never sell then that's often the deciding factor, but I'd still consider it before you do- especially regarding the outside of the locks. Once you cut through that patina there's no going back...

If you're going to try finding or fabricating blanks then it's worth going all the way with restoration as there;s a lot of intricate work involved anyway as oldlock says. Painting looks fine if you can replicate the right 'look' and finish to it- there's a company in the UK called Frost Automotive Restoration which used to do an amazing range of kits for painting and electroplating different finishes, and amazingly they did a Black japanning kit- the only one on the market I've known of. Don't know if they still do them though, but it was in the last catalog i had a few years back.