Vintage desktop Remington Rand 10 5/8 inch card index for 3 x 5″ cards 
boffosocko.com
boffosocko.com
@chrisaldrich These vintage card index boxes look great. 4x6" FTW though. Are the 3x5" boxes easier to come by?
@writingslowly For boxes of this size and type 3x5" are far easier to come by. In the 6+ months I've been watching various searches I have yet to see one of these for 4x6" cards. (I'm half tempted to buy a 3x5 box, cannibalize the hardware and build a 4x6" box. 4x6" boxes are easier to find in wood in larger 4-6 drawer form factors or much more ubiquitous in the metal multi-drawer (2 or 3 drawers) stackable form factors from mid-century to the late 70s. For more options, see The Ultimate Guide to Zettelkasten Index Card Storage .
@chrisaldrich thanks - your 'ultimate guide' was appropriately named! It's interesting how much of this old-school tech has been ditched since the 1980s.
@chrisaldrich In Australia index cards are often called 'system cards ', but no one says which system is meant. There's a few candidates, e.g. bridge playing or even the Memindex system. The system is being reinvented by Facebook.
@writingslowly Thanks Richard. I've also just noticed that I know you by a separate handle over on Reddit. Happy to put the two identities together.