Combro issue

[ashf9999]

The £20 ones you bolt on?

People show off a test of theirs quite regularly. They work. They're not calibrated but neither was your combro and neither is almost every other chrono out there.

Good enough for hobby grade

If you mean not calibrated, you mean to something like a registered/recognised standard? All chronos would have had some form of calibration check during manufacture/set-up. They just don’t have a calibration certificate issued.
Anyway, I thought later Combros could be user-calibrated against a known source if you wish, as they came with the instructions on how to adjust the readout.