Brother KA-800 is a mystery carriage: it is not mentioned anywhere. Yet, I found it on an online auction.

It looks like a better-known Brother KA-8300 transfer carriage.

I see some differences in the display pictures on the carriage itself, but I do not see any differences in their functional parts.

There are videos on how to operate it. Basically, one positions the carriage on the main-bed+ribber setup following the diagram shown on the carriage and pushes along the needlebed so the stitches transfer. A lot of people think that these carriages are finicky. Often, the last two before the transfer are recommended to be knitted on a looser tension.