If the spindle is a simple on/off, then you won't need an analogue output.
However the top left board looks very like a KB drive of some form, and there are wires connected to the terminals that would normally have a speed potentiometer connected (they can be controlled either with a potentiometer, or via an isolated 0-10V feed). IIRC the ID/Model label would normally be on the face of the heatsink pointing towards the side of the case/opposite side of the bit where the SCRs are bolted on.

And having just checked the wiring diagrams ( Denford Software & Machines • View topic - MicroRouter Compact 2002 - 2004 Circuit Diagram ), it does use a 0-10V analogue signal. The Baldor controllers that Denford use, are pretty good things and have quite a bit of Inputs and Outputs, including analogue. It's just a shame they cost so much. I think the cost of an upgrade from the model in your router, to the lastest USB version is over £1200. I happen to have the latest USB version in my Novamill, and I've got no plans to swap it as it just works with no issues, and I've just added a coolant relay with minimal hassle.

Another issue I notice in the wiring diagrams, is the micro routers don't have a 24V DC supply. They simply use 24V AC relays, and feed 24V AC into the Baldor, which has it's own onboard rectifier/regulators. Both my Novamill and Cyclone have linear supplies, so they must of gone for the cheaper option when doing the micro routers.
That creates the issue of getting a stable 24V DC supply for any new controller. Cheap option would be a 24V DC din rail switched mode supply, but the ideal option to minimise noise would be a 24V DC linear supply.
Rectifing the existing the 24V AC isn't that viable an option, as you would still need to regulate it down to a consistent 24V DC. AC stuff is usually far more tolerant of voltage variations than DC, which usually has pretty strict voltage tolerance.