From 1926 an instruction was issued to preserve a similar number of 11 ton trucks, of which there were about 6000 at the time. To accommodate this, light channel side 16ton I wagons (ie 'Tommy Bent' wagons) were to be marked down to 11 tons, dual lettered '16 or 11 tons' and a letter 'A' added after the number. By late 1926 this was found to be unworkable due to lack of space for the 'A' and not being visible at platforms with the doors open. The position of the 'A' was changed so that it appeared after the code. At present this appears not to be a class had not changed and the 'A' became an indicator of traffic use.
It seems that only from 1928 was the indicator formalised into the code creating the class 'IA'. By 1929, the railways split the _I__ wagon group into subgroups which allowed for easier capacity recognition. As older lower capacity _IB_ wagons were scrapped an equivalent number of I wagons were recoded to IA. This was a concession to farners who complained about ordering a vehicle for say a 6 ton load and were required to pay a higher rate because a 16 ton vehicle was provided. Wagons were reclassed from I to IA, about the rate of 50 to 100 per month; the same rate as the scrapping of 8t - 11t wagons coded _IB_.
As with the _I__ wagons, the class were extensively rebuilt and modified for different traffic. Some of the rebuildings were to _IC_, coal; _IT_, timber, _IK_, flat wagon; _KW_, woodchips; _KF_ / _KW_, new car bodies; _HD_, service vehicles for store wagons; _HR_, workshops transporters; and others.