The HP 9810, 9820 and 9830 were all announced simultaneously by HP in the December 1972 HP Journal (picture). The 9810, 9820 and 9830 were all four bit machines and all of them used a serial bus internally.
The HP 9821 was first listed in the 1976 HP catalog, although it was previously advertised in an ad in (picture) the January 20 1974 issue of Electronic Design News. The HP 9821 was a HP 9820 with a magnetic tape drive replacing the magnetic card reader. These seem to be very rare, I know of a total of four of them. I have never seen or heard of a manual specifically for this model. This model is the same size and weight as the 9820 and has the same capabilities. It was listed as having only 167 registers instead of the 173 that were available in the HP 9820. The firmware for the cassette drive used the additional six registers.
HP 11220A Peripheral Control I $485
HP 11221A Mathematics $485
HP 11222A User Definable $485
HP 11223A Cassette, ROM $225
HP 11224A Peripheral Control II $485
