Looking at the examples for external memory OTA for BLE : on the BLE component, in the Bootloader Service, Command Characteristic the Data field is set at 137 .
How is the right figure calculated? I couldn't pinpoint a reference that explains it. With a different project, keeping the same value 137, the firmware transfer fails with a "write long characteristic value timed out" message. I suspect the error has to do with being the data field of the command characteristic in the bootloader service not right.