The second and third variant were used in the Model 2000HD and available seperately. One had both an external connector as well as provisions for attaching ribbon cables to run internal drives. Some enterprising users even ran cables from the external connector back into the case so that two half-height hard disks could be mounted internal, as the board didn't support the standard daisy chaining of multiple drives off of the internal cables. The other, which is the earlier version of the two, had only the internal connectors.
Tandy also sold a full upgrade kit for dual-floppy Model 2000's. This included everything needed to add a hard disk to the Model 2000, including the controller board and a 10meg hard disk, and carried part# 26-5125. In 1988 with my original Model 2000, I gathered up all the peices needed to add a hard disk to a dual-floppy machine and used a 20meg ST-225 in the machine.