The model type (I assume that is what it is) on the gearbox in my 2.2 says "AC 506". I have heard that it should be a Citroen CX box, but I have been unable to find any references to AC 506 in connection with gearboxes.
That's strange if you have a '506' in a 2.2. The 506, 511, 2GE14 and 2GE16 were all Murena 1.6 units whilst the 510, 2GE15 and 2GE17 were fitted to 2.2 models. The differences between the two groups was the differential ratio. I wonder if someone has fitted a 1.6 unit to your 2.2? The only other explanation I can think of, is that the box itself was nominally marked 506 but that the complete unit would take on the correct designation after it was assembled with the diff. However, I have never actually seen any designation tag on any Murena transaxle, so I don't know where it would be found.
According to the information I have, 506, 2GE14, 510, and 2GE15 are all Lancia units, whilst the others are Citroën.
All the Murena gearboxes have the same 1st to 5th gear ratios, and the only difference is the diff. ratio - 62:13 in a 1.6 and 61:14 in a 2.2 (they are the crown wheel and pinion teeth numbers) The gear ratio set is not the same as any other Citroën and I have records for I think all the models including the commercial vehicles that also used these units.
The 1st and 2nd are the same as some but with different 3rd, 4th and sometimes 5th. Some have different 1st and 2nd, and the same or similar 3rd and 4th and exactly the same 5th, but the diff. ratios are different. Part of the reason for so many differences comes from the fact that Citroëns usually had much larger wheels and tyre rolling radii than the Murena so they required a different figure to give the right overall gearing.
That was how I came to realise that if I changed the 5th gear to that used in the CX GTi Turbo and Diesel models, it would make an ideal 5th for the Murena.
So the first thing you need to find out for certain is, exactly what diff. ratio is in your unit. Since your unit should be a Lancia version, it simply means it came from an Italian factory rather than a French one I think. The parts microfiche details where the internal parts used differ.
What do you need to know? My own gearbox parts list is fairly comprehensive one as I have combined all the Matra and Citroën information over the years. Many of the items are simply no longer available anyway, and others take an age to obtain - it took me seven months last year to get some bits!
The ideal box to get from a scapyard is a diesel or GTi Turbo unit as it has the best set of ratios including the high fifth for a Murena but the diff ratio will be incorrect. However, these are like 'hens teeth'! So good luck on finding one.
Roy