I got more metrology toys this week. I found this set on ebay. The price was fair for a complete set (1, 10, 100, 1K, 10K, 100K) so I sent a really low offer. Seller just accepted it
In the end I paid around 16$ each resistor.
They are NBS (National Bureau of Standard) style secondary resistor standards from Leeds and Northrup. The resistor is immerged in mineral oil and encapsulated in a metal container. To improve thermal dissipation they can be optionally immerged in an oil bath.
I measured all of them and they look good. The oil need to be changed though. It's supposed to be transparent and is now kind of brown.