@thomasfuchs Good thing they are labelled, I would have assumed they were MIDI ports.
@starlily The most common use for DIN3/DIN5 ports was actually analog audio. (Single-direction analog stereo audio only needs three leads, but the most common DIN5 sockets allow insertion of both DIN3 and DIN5 plugs, so as per Postel's law, DIN5 sockets and DIN3 plugs were common.)
Somehow, 3.5mm TRS connectors became more popular in consumer audio than the DIN3 plugs, though, so much so that nowadays, people seeing DIN3/5 connectors often mistaken them for XLR connectors, which look similar and are used for similar purposes, but are mechanically incompatible — partly, because they come from USA, so they're drawn up using dimensions that seem round to an imperial measurer.