Pictures attached of mine, opened and closed... not quite the same. The slide bar doesn't have the rod that the handle pulls visibly welded like yours but its the same concept.
I would use something a lot more heavy duty than WD-40 to lube it. I use Valvoline Cerulean (lithium complex NLGI-2 - basically a wheel bearing grease) and use a paint stir stick to dig it out of the tube and slather it on.
My 5er has the Reese Revolution pin so I don't use a disk (it doesn't pivot on the hitch). The grease on the head is just for sliding while hitching and unhitching.
Good to see that you use a lock - never move without it.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam