Not sure of this is of interest to anyone, but I thought I’d pass it on anyway: http://www.ebay.co.uk/itm/300709037063.

It looks like a stock image on the auction, but I found what looks like a higher resolution photo of the same keyboard.

I know absolutely *nothing* about this keyboard, and I can’t find much about it online either. It’s mentioned on this Unofficial DEC Documentation page, there’s a little about the machine that used it on this page, and it’s covered in this dump of some Usenet threads.

One of these were on ebay.de with a few other keyboards, didn't sell a few times at 1€ and there was another at 5€

You can get them cheaper
http://www.ebay.de/itm/140476199068
http://www.ebay.de/itm/261021515189
http://cgi.ebay.fr/itm/251057571019

Any idea if the caps are mx compatible ?

LK401 is a horrible mushy rubber dome keyboard. The key caps are slanted and have a curved surface. The key caps are not MX-compatible, as the slider touches the dome directly. I have one in Swedish layout that I'll gladly donate to someone who wants it. £49.99 is grossly overpriced IMHO.
The key caps might be PBT, but I have not done the acetone test on them.

Edit: Wiki stub added.

Thanks for the replies. I found a page about the VAX machines with which these were used that makes pretty interesting reading.