Mine are the ones with a round base and a forward facing tab, internet pic attached (not my actual bosses but they're the same). The standard is the campag type, with a square base. The curved washers on late 80's - early 90's shimano downtube levers are for converting the campag type braze on bosses to shimano type.
I know the batavus was originally sold with a shimano 600 group, which I guess is the reason for the shimano braze on bosses. I hope to fit some campag levers but they won't go on without some fiddling or fabrication.
Or I might just have to fit some shimano levers :facepalm: