BNC or AUIA Sun 3/50 or 3/60 is equipped with both an AUI and a BNC ethernet port. But how can i switch between 'em?
If you take out the mainboard you can find a dip switch block near the ethernet ports. Flip all eight switches into the direction of the desired port and you're done.
On the mainboard near the battery is a set of eight jumpers. Six of 'em configure the memory size (4M,8M,...,24M), one changes the video resolution (HIGRES) and one controls the ethernet ports (EXTUR). If you pull this jumper, the BNC port is activated.