AFAIK, there is no special firmware.  Early 3458A's did not have the VHP101
reference resistor, but these were (silently) added by default in later
models.  So, later models will have much improved resistance specs in some
of the ranges.

Not all VHP101 resistors are the same.  Some of them are so good that
their TCR is loss in the noise when trying to measure it.  Most of them
have some measurable TCR (both Alpha and Beta).  So, you would have to buy
10 (or so) and perform the measurements yourself.

Another difference in the HFL was that they had the most stable reference
boards installed.  All reference boards are built, and monitored during a
very long burn-in period.  The reference boards are graded, and the ones
that drift less than 2ppm/a are the HFL ones [p/n 03458-66529], and the
ones that drift less than 4ppm/a are the "option-002" ones [p/n
03458-66519], and then the ones that drift less than 8ppm/a make it into a
standard 3458A  [p/n 03458-66509].  The rest of the reference boards are
left to burn in longer (or are scrapped).  They probably look at the 1/f
noise of the references, and any that have substantially larger 1/f noise
(compared to some historical norm) are scrapped even before they go into
burn-in.  If you have a very old 3458A (say at least 10 years old), it is
likely that the reference in your unit already meets the 2ppm/a HFL specs,
and so this board is not needed.  You can also "pimp" your reference board
by adding a 100K resistor across the 15K foil resistor [I would use another
foil resistor for this].  This lowers the reference temperature for
laboratory use only, and the reference drift can possibly achieve better
than HFL specs.

The final difference in the HFL is that it had some pre-programmed function
keys, with functions that are used very often by metrologists.  There was a
stick-on label that went over the keys with the new functions indicated.

There is a document on the Fluke Calibration website that describes all of
these differences, and it even tells you how to add the special
pre-programmed functions yourself:
http://assets.fluke.com/manuals/3458ahfliseng0000.pdf

So, that's about it.  It is very likely that your unit already has a
VHP101, but it may not be a "highly selected" one with near-zero TCR.  A
standard VHP101 is pretty good even if not selected.

Hope this helps!

Regards,
Ken

A friend gave me a dead Solartron 7060G (TRMS version) It goes on with
al segments and all leds on.  A quick probing shows the -35V measures
only  -20V. I have not probes more rails after that.

Does anyone has a service manual for this meter.

Fred