what's the reinforced hose with the circlip do? it's jutting out of the tailpiece by the tail-light in the last image. maybe a crankcase breather or tank vent?
Catch can overflow, usually on the older well prepped racebikes the overflows for coolant and crankcase would route to a aluminum (purpose built) catch can under the seat mounted to subframe. It would usually be full of steel wool or other sponge like material to prevent sloshing and it would be vented to a tube like that that came out the tailpiece. The R clip holds it in place so it doesnt back itself out and point down to the rear tire. Club racers often use Nalgene bottles with holes drilled in their caps or beer cans sealed with RTV. Modern era they are often routed to Nalgene style bottles mounted to the front fairing stay.