I noticed this googling part numbers out of the 3478a service manual,
a Chinese thread where people were discussing replacing the reference resistor with a more stable one.
(Their words not mine).
Is this sort of mod worth doing? It seems to me like it might be problematic.
But i don't know if it's worth doing since today's resistors might be made to tighter tolerances.
Any thoughts on this?
In the 3478 the relevant resistors that set the Ohms mode scale factors are all part of the hybrid(s). So there is no way to replace them.
Anyway the 3478 is a relatively good 5.5 digit meter - not one to get really highest performance. It may share the input hybrid type with the high grade 3457. So I would not worry much about the ohms accuracy.
The part shown between the 2 relays is a capacitor responsible for AC measurements.