TLDR: OP wants TO-92 depletion mode JFETs suitable for replacing the current shunt selection JFETs in a 3458A.
Googling for 1855-0743 finds
https://xdevs.com/doc/HP_Agilent_Keysight/3458A/doc/3458A%20CLIP.pdfWhich lists description as "TRANSISTOR J-FET N-CHAN D-MODE TO-92" and matching manufacturer part numbers as NSID5092/F50188A . But then the trail goes cold for me, at least as far as finding matching datasheets is concerned.
However, looking at the schematic, these transistors are almost connects to nets with guard rings, which implies that extremely low leakage current would be a requirement. Given that this is a multimeter, high-frequency performance seems unimportant. So here's a shortlist:
http://www.digikey.com.au/products/en/discrete-semiconductor-products/transistors-jfets/288However, the parametric search here doesn't help with finding JFETs with exceptionally low-leakage at cutoff.
Does anyone else know of parts specially designed for this purpose? Also, I can't be bothered tracing the nets through to see what acceptable threshold voltages would be.
Assembly 03458-66501
Q201-Q206: p48
Q107-Q108: p49
Q12-Q13, Q15, Q18, Q24: p50
Assembly 03458-66502:
2 others, didn't look into further.