The information would probably have come from
mg-rover.info, but that site disappeared from view some days ago - I am led to believe that Steve Childs is in the process of switching to a different host, but nothing has been said officially, so we will have to wait and see whether it re-appears or not.
Also, I would treat the figures in the TF section on it with a degree of caution; either the build records were not filled in very accurately in the first place, or whoever sorted the MG Rover database out when the computers at Longbridge which held them came to light (several years after SAIC took Longbridge over) didn't get things quite right. The build records were I believe somewhat corrupted, and whilst there are not many errors or anomalies within the records for the ZR, ZS, ZT and Rover 75, the records for the TF are very haphazard, and many of the numbers for the different versions and colours are/were incorrect. This was especially true amongst the Monogram coloured TFs - not helped because a great many Monogram coloured TFs simply had the paint code for Starlight Silver (MBB) stamped on their VIN plate, and probably recorded as such on the build records.
If you want accurate information for a TF, it is probably best to try contacting Adrian Clifford - he has posted in here occasionally, but can mostly be found on faceache.