Just an update – I found pictures of the MS150.2 crossover online and it appears that mine have been half re-capped! However, one of the original capacitors has broken and one of the replaced caps appears to be the wrong value.
Consequently, I have done what any sensible person would do in this situation and ordered a full capacitor kit from Beoparts!
Would still be keen to know if there’s a service manual out there, though.