The factory default hardware confirmation of ZD621 printers does not include the Bluetooth classic. Therefore, the 7936 is returned from the bluetoothClass.getMajorDeviceClass(). If the ZD621 has the Bluetooth classic equipped, then the 1536 (0x600 for IMAGING) will be returned from the bluetoothClass.getMajorDeviceClass(). To verify if a printer has the Bluetooth classic equipped or not, use the SGD of the bluetooth via the Printer Setup Utility for Windows tool. In the response, the item of the bluetooth.le.controller_mode shows what type of the Bluetooth is equipped and enabled, i.e. the Classic, LE or both. Probably the ZD621 printer you have only has the LE.

### Bluetooth SGD
! U1 getvar "bluetooth"

<< example response >>
bluetooth.
bluetooth.discoverable : on , Choices: on,off
bluetooth.friendly_name : XXZJJ174600934 
bluetooth.version : 6.2 
bluetooth.date : 01/01/2020 
bluetooth.local_name : XXZJJ174600934 
bluetooth.address : AC:3F:A4:CD:C5:15 
bluetooth.bluetooth_pin : * 
bluetooth.short_address : AC3FA4CDC515 
bluetooth.radio_version : 5.2 
bluetooth.enable : on , Choices: on,off
bluetooth.connected : no 
bluetooth.minimum_security_mode : 3 , Choices: 1,2,3,4
bluetooth.connected_security_mode : nc 
bluetooth.bonding : on , Choices: on,off
bluetooth.clear_bonding_cache
bluetooth.enable_reconnect : iOS_only , Choices: off,iOS_only
bluetooth.le.
bluetooth.le.controller_mode : both , Choices: both,classic,le
bluetooth.le.power_class : 2 , Choices: 1-2
bluetooth.json_config_channel_enable : on , Choices: on,off
bluetooth.page_scan_window : 60 , Choices: 15,60
bluetooth.power_class : 2 , Choices: 1-2

To detect if the paired Bluetooth device is a printer or not, use the BluetoothClass.Device class. Refer to the Bluetooth Class Device & Service Generator to generate the const for comparison. For a Bluetooth printer device, the Major Device Class is IMAGING, while the Minor Device Class is the PRINTER. So the combined value is 1664 (or 0x680). Use the following comparison to determine if it's a printer or not.

if (bluetoothDevice.getBluetoothClass().getDeviceClass() == 1664) {
    // A Bluetooth printer
} else {
    // Not a Bluetooth printer
}