HP-19C
HP-20
HP-25
HP-29C
HP-67
HP-97
But, the hardware differs (buttons, I/O ports, etc) and can cause software to be unusable but the software is freely interchangeable. Games were made specifically for each but often work on other models. Sometimes the same game was published separately for two or more of these. These models use HP's magnetic strip storage so sharing programs is as simple as floppy disks. HP's magnetic strip storage is also used for several other HP calculator models that are not compatible.
HP-65 is different from inaccurately called "20-series" listed above.