I thought that all European vehicles ( apart from the UK ) were left hand drive.
Far from it. France has always been right-hand traffic¹, so it’s odd that the Lafflys had the steering wheel on the right rather than the left — and I can’t think of a reason why. However, for example, Austria gradually switched from left-hand traffic to right-hand between 1921 and 1938 (the
Anschluß was the last push needed to fully switch), Greece switched to right-hand traffic in 1926, Italy over the course of 1924 to 26 (after Mussolini decreed this in 1923, as before then, each province decided for itself, so both were used), Sweden in 1967, etc.
A good read on this is the article
“Left- and right-hand traffic” on Wikipedia.
Was the Laffly an oddity, or were RHD vehicles common?
Not sure about France. They were in Italy, as Italian manufacturers of trucks and similar often kept making them with the steering wheel on the right even after the whole country had been made to drive on the right. You see that a lot in Italian military vehicles from the Second World War.
¹ Note I’m talking about right-hand traffic here, which generally means left-hand drive.