2a/3 hybrid - advice sought

1st post so be gentle!
I recently bought my 1st 'real' landy, a 109 diesel pickup. It's a series 2a but from the bulkhead forward it's a series 3. It appears to
be an ex-military as it has a separate panel for the lights (with HST,S,T,Conv etc) and the battery is where the middle seat should be. There is a large metal cover over the battery with a warning of 'danger acid' on it. Most of the wiring is absent so none of the gauges work (I've got a series 3 wiring diagram so I'll attempt to sort that lot out). I cannot find the engine number though. I have looked where it should be (according to the manual) but there is nothing there. The only number I can find is on the side of the engine under the exhaust manifold. This has the letters 'AN' underneath which are the numbers 541874 (I had to use a torch and mirror to see these). Does this mean anything to anyone?
I would also like to know if it has a 2a or 3 gearbox. Will the gearbox number (assuming it has one) indicate this, is there a list somewhere which will tell me?

Thanks in advance for any help/advice
rileyrma (now there's a car)

The gearbox is easy to I'd first where is the clutch slave cylinder
If it's drivers side of the box and pointing vertical it's a s2 clutch
If it's pasangers side mounted horizontal then it's a s3 clutch
But the gear box it's self may not be what the clutch indicates
If the gearbox has synchromesh in only 1st and 2nd then it's a s2
But it there is syncromesh in all gears then it's a s3 main box
My s2 RAF ambulance has a s2 clutch but a s3 main box they are a hybrid at the end of the s2 production but the 2 different systems work perfectly together and when ordering any parts just treat as a s2 or s3

Thanks for that, I'll check it out tomorrow. A guy on the LRO forum reckoned that the chassis on a series 3 109 was seam welded along centre top and bottom and that series 2as were not. Is this true? If so it will help me decide if this is a series 3 masquerading as a 2a.

rileyrma