The original machine, as built by Booth at Birkbeck College was variously known as the Hec (Hollerith Electronic Computer) and as the 1200.
When BTM took over production, the first models they delivered were still designated as Hec but with the number 1201. The card-reader/line-printer unit on this production machine was based on the JRTT (Junior Rolling Total Tabulator) or SRTT (Senior Rolling Total Tabulator) depending on customer choice - the main difference between these two being number of print positions (width) of the printer.
A year or two later, BTM came out with a new line of tabulators (the 900 series) which were much more flexible and powerfull, and a new new model of the Hec was introduced - the 1202 - using a derivative from this newer line of kit for its card-reader/line-printer.