Could you give me a reference for your assertion that the "Centeurs" were reduced to a battery of 8 within two weeks? Considering that they were still being used in mid-August and all that.....
I don't have time to dig through the files and pull together the references. The the story can be pieced together from the War diaries and correspondence in HQRA 2 Army, unit histories and the report on their effectiveness by the OR Group (report no 2).
The RMASG was established as an assault unit purely for D Day. It consisted of 20 troops @ of 4 x Centaur CS and 1 x Sherman organised into two regiments and an independent battery. These evolved from the plans to have 17 Pdr turrets fitted to landing craft to provide fire support during the last stages of the D Day assault. The detachments were formed from RM gunners, from the artillerymen made available through the disbandment of the RM Division together with RA commanders and RAC drivers from the reinforcement pools. It was a disposable SP Brigade and intended to be disbanded as soon as the landings were complete. The Navy were very sensitive to the risk of their marines becoming absorbed into 2nd Army by default and the enthusiasm of their detachments and tried to impose restrictions on how far they were to be used inland. The unit had no administrative or maintenance staff or even any B vehicles as it was not intended to exist after the assault landing succeeded.
After the assault landing several of these troops attached themselves to RA units and became "extra guns" for some days after D day. During the following weeks the majority of the manpower and reusable equipment was returned to the UK or the relevant reinforcement pools. A minority was retained and re-organised as an SP battery, with command posts and enough B vehicles to sustain themselves and deployed in support of 6th Airborne Division East of the Orne. it's initial use in a counter mortar capacity was hampered by the low trajectory of its armament. The unit was disbanded shortly before the break out from Normandy and IIRC did not take part in the advance by 6th AB Division to the Seine.
The limiting factor on the reduced size of the RMASG was manpower and supporting infrastructure.. The unit was engaged in a largely static role after D Day and had no integral REME support. No one seems to have been interested in recording the mechanical reliable of this obsolete equipment per ce.
The story of the RMASG does not tell us much about the reliability of the Centaur as a tank, apart from the fact that the vehicles were considered disposable in mid 1944.