The only difference between the A1200 and the A1200HD was the addition of a 2.5" IDE Hard Drive, plus a few extra disks used for hard drive prepping and installation. There is a lot of controversy of what sized hard drives were supplied with the A1200HD by default, however it's been confirmed that American and Australian A1200HDs were supplied with 40MB Hard Drives whilst European models were sold with 20MB hard drives. Apparently the reason why Commodore supplied different hard drive sizes was because the European market was primarily geared towards games whilst the American and Australian markets were mored geared towards productivity and therefore required larger hard drives. The Australian (including New Zealand) and American models carried the A1200HD/40 designation but it is unknown if the European models carried a similar designation or were simply labelled as A1200HD.
It was quite common for dealers to pre-fit standard A1200's with varying sizes of Hard Drives which may have lead to some confusion over what size hard drives the A1200HD had by default.
Please see the Commodore A1200 and the Amiga Technologies A1200.
Thanks to Marius Lauritzen, David Woolley and James Jacobs