No guesses so another image.
Must be a rubidium frequency standard from a military navigation system. I wouldn't be too surprised if there is an Efratom or Ball Aerospace-Efratom rubidium clock inside the box.
Almost right. It's for a secure frequency hopping radio. I connects to the Have-Quick enabled PTR-1751WWH
It contains a Frequency Electronics FE-5650A-M
The output is 800kHz divided down (by 16) from 12.8MHz generated from the interal DDS. As the 12.8MHz is crystal filtered it is not easy to change to 10MHz even though the 5650A has DIP switches internally. It can be done but 800kHz will easilly divide to 100kHz and 1Hz so still useful.
Anyway worth £20 each anyday.
Note I've not put the number etc. I got burnt a bit on these years ago. Some turned up on ebay I wasn't placed to buy more than one. Someone else bought one and asked about it on the time nuts mail list. I told him what I knew and net thing he had bought them all, taken the Rb's out and sold them at a large markup.
Scotttrade12 has done similar recently. The first listed them complete at silly monery then dropped the price to £20
www.ebay.co.uk/itm/224434964973? but that is the EMPTY CASE. He sold the Rb's for atound £100
www.ebay.co.uk/itm/324574645606.