Chronometer certified Longines 30L and 30LS movements.
Longines made a few hundred movements, equipped them with a special balance and had them certified as Chronometres. It was also made a special version for competition at the observatory.
Quite certainly this was done for marketing purpose, to be able to show that the 30L movement was “Chronometer material”.
Needless to say, these watches are rare today.
Often, it was placed in a Flagship case, and quite a few found its way to UK.