Thank you very much for your answer.
We shall see if second calibration lab is able/willing to do what could be needed, and how much money they ask for. First one was not.
I already ordered one each of 100, 1K, 10K and 100K Vishay resistors, Z and S series, 0.01% and, supposedly, 1-0.2 ppm. 28-36 € each, plus VAT. For the higher Ohm ranges, just looked for the best tolerance/ppm in stock on Digikey right now. These higher range resistors are way cheaper, about 4-9 € each, plus VAT.
I know these are neither metrology-grade resistors nor what HP3456 manual calls for. Far from it. However I'm not willing to spend more on "standard" resistors. I think there's no need to go further, taking into account what my skills are and what I do as a hobbyist. I just hope they will allow me to see if Ohm ranges are out of specs right now. At least lower value/pricier ones. I also hope, at least the pricier ones will be minimally stable to be used again in the future.
After checking resistance ranges, I will make the decision about sending the HP3456 to calibrate/adjust, if any calibration lab finally sends a quote to me. I already have a DMMCheck. It can be sent to welectron in Germany to be calibrated again if needed. That way, I'm quite confident I'll be able to see if any of my "standards" gets out of whack in the future.
If I understand it right, HP3456 has a LM*99 internal voltage reference?. I don't think DMMCheck LT1021 is better than that. So a better voltage reference would be in order. Not sure about making a LM399 reference, it probably wouldn't be good enough?
Checking against DMMCheck, my HP3456 seems to be quite good, just the 100 Ohm range looks bad, at 10 PLC 2-wire, 5 1/2 and 6 1/2 digits.
I take note of what you say about resistance bridge/box. Something like that if I'm getting it right?
https://www.ebay.es/itm/256087451550? https://www.ebay.es/itm/175876283467?Problem is, not only shipping from the US is crazy expensive, but that US sellers doesn't seem to like eBay's international shipping *program*. In that case, we EU buyers are also extorted a customs paperwork fee (about €50 in that price range), and, on top of it all, VAT. Far, far away from getting it on a hamfest. But I will look for it. Perhaps a bargain will appear, who knows. I already got an ebay search at work
Even if there's no bargain bridge/box, it could still be cheaper than buying about seven hundred resistors to match about 30 of these.
Last, this is what i got for 4-wire connection, I still have to do a BNC-banana adapter for HP3456 and DEER EE-5000, that stuff is lying around in some drawer right now.
https://aliexpress.com/item/32505106470.html?https://aliexpress.com/item/1005004235132283.html?Thank you again
Edit: link