Looks like the interface connector is made by Glenair.  Those connectors are similar in cost and reliability to the infamous Amphenol ones used in most other military equipment.  They are not unobtanium, but they are not cheap either.  I agree with you about the large IC;  Probably some kind of 8051 part, custom ROM masked for this application and house numbered.  If you really want to print something with it, you may find it easier to reverse engineer and duplicate the drive circuitry for the print head and associated mechanism, and using a microcontroller of your choice to handle the interfacing.  There may be some kind of key exchange, or unlock that has to happen to enable the device. I could also be totally wrong, and you may find it just responds to serial data on the four-pin header using  some kind of simple serial protocol like some older receipt printers.  There does not seem to be any sort of voltage regulation, so I bet you supply it with 5V directly, via the four pin interface connector.  I notice a cable coming off to the side of the Large IC, what did this hook to?

I can't give any exact details because it's in my office and I'm still in COVID lockdown.

I've already chopped the connector (I was thinking "not unobtainium but not cheap either myself)  off with an aim of fitting a D-plug instead (hopefully) based on the pin outs of the  MAX232CPE... I hadn't thought that there might be some kind of unlock key exchange... that will be annoying if there is.

It's power supply came from (maybe) 6 or 8 AA cells.

I'm pretty sure (would be so much easier if I had the unit here and wasn't just looking at the photos and guessing like everyone else) that the bundle of wires next to the big chip is for the power... that doesn't seem right... you'd expect just a red and black... but I am fairly sure that's what it is.

I'll be able to give you a straight answer (or at least a "strighter" one) when I get into my office.

Any chance you could remove the stickers from the LA-180D.  Curious if there is anything markings underneath them.  Also, see if you can tell me which pins are power and ground? 

Curious design.  With all the TO92 transistors on the pcb, and without knowing the layout, I'd guess that the LA-180D is some type of processor and the transistors are for driving the printer.

I can't get into my lab at the moment because of Covid-19 so I can't answer any of your questions right now.

I don't think it's that likely there will be any markings under the barcode just because it's more usual to put them in the middle of a DIP package... and there they are: "LA-I80D 46 062028" but I can still rip it off and have a look for you.

I hadn't bothered checking any of the pins of that big chip, which I too am assuming is some kind of processor or microcontroller. As I had found that that the Maxim chip is a RS232 driver, I was planning on working from there with a known chip with a datasheet.

The M-180 printer module has a separate line fro each of 7 solenoids and a motor so it should need a few driver transistors... but still 21 does seem to be quite a lot.... but military stuff is kinda well known for being hideously over-enginiered (??????)

One thing I can do without being anywhere near the unit itself....... I've just added a link to what is almost certainly the datasheet for the print module itself.

I'm just uploading a picture now... sadly there was nothing under the sticker.

See: https://hackaday.io/project/168472/gallery#64f1611a6b6d3fba3fc8634322f843fe

I'm still nowhere on finding power and ground... There's no obvious high-current or ground-plane connections going to it at all... I'll have to power it up and probe it for that. And that could take some time for reassembly. Sorry this is taking so long but I'm still only going into my office a couple of days a week at the moment.

What does it print? Surely not dogs?

I mean what kind of material does the printing mechanism suggest? I see some gear wheels, what are the mechanics? What kind of ink or marking substance is used? Maybe if you looked into those aspects rather than the electronics first you might have an idea how it works.

The other thing is it being army surplus perhaps you should be asking in forums where people who have used it could tell you what it does.

It uses an Epson M180 print module, I think. The module certainly looks identical to the Epson m180 series. This is a traditional dot-matrix mechanism with a miniature ribbon.

I've been around some forums but as it was part of a secret communication system, even though it's mechanism is very straightforward, the equipment it was connected to was covered by "The Official Secrets Act" and can't really be talked about for 100 years.