Watched the Lupin The Third 50th Anniversary Special last night- it appears to have been made between the 4th and 5th seasons(?). It's this kinda-sorta remake of the very first episode of Lupin, from the last 70s, that then changes as Lupin gets sent to various eras from previous anime seasons. It probably would have helped if I had watched it all, because I had no idea what was going on.
Still fun to watch though!
Was going to move onto Goemon Ishikawa's Fountain Of Blood next, but just found out that it's the second OVA in a series following The Fujiko Mine anime series and that there's a Jigen Daisuke OVA that came out first, so downloading that now. I watched a bit of the Goemon one and it's super-violent, which is weird for Lupin.
AFAIK these OVAs and the 50th special are only available fansubbed.