1920 CL stock on the Central london

Bear with me - I have a vague memory of seeing something in the traffic notices. Can't promise, but I shall have a root around,

In 1926-1927 the Central London Railway was short of stock because of the reconstruction programme of its trains was at its height. This involved the modernisation and the fitting of air-worked doors to all CLR stock. To enable services to be maintained on the Central, two six-car trains of 1920 Cammell Laird stock (with the Piccadilly Line converted motor cars) were loaned to the CLR from mid-1926 to the summer of 1927. This disposes of many arguments about the ability of normal tube stock, almost 9ft 6ins high, to traverse the (then) slightly smaller Central London tube tunnels. CLR stock was but 9ft 4½ins high. It is possible, however, that the 1923 and later tube stocks 8ft 3ins wide at the eaves, would not fit, but with its 7ft 9ins eaves, the 1920 stock would! However, a concession was made. The 1920 stock was 9ft 5⅜ ins high with new 30ins diameter wheels – the wheels were 26½ins diameter when regarded as ‘fully worn’. The cars which were sent to the Central were fitted with 28ins wheels, hence were 9ft 4⅜ ins high on the basis of this one-inch reduction in wheel radius! Nevertheless, we learn from LER Traffic Notice No.45/26 (November 1926) that the 1920 stock was banned from using Marble Arch siding. By examination of the car mileage cards, we see that the mileage actually worked on the Central London was relatively low, in comparison with traditional CLR stock, which suggests that the 1920 stock may have been used only as a last resort if no other CLR stock was available.
No other modification was needed to the trailers. On the motor cars, the only modification necessary was the removal of the positive shoes and connection of these shoe leads to the bogie frame (or connection of the main positive cable to the car or bogie frame). The stock then worked reverse polarity with its negative (centre) shoe on the CLR positive rail, and track return – indicating perhaps a reluctance for them to be used freely?
Two three-car sets were transferred from the Piccadilly to the Central in June and July 1926 and a six-car block train in September 1926. The two threes went back to the Piccadilly on 1 July 1927, the block train on 1 September 1927.

Thanks Reg. I often wondered why the CL stock was Out of gauge on the GN&C.

Just a wild guess here, but is it something to do with carriage lengths and sweep profile?