The 4815A used P channel FETs which were available 50 years ago
and are now unobtainium.

The 4193A used N channel FETs which were available 10 to 30 years
ago and may even be currently available.

They are DEFINITELY NOT INTERCHANGEABLE.

This is according to ex-HP'er George Standford, who used to
support vector impedance meters with HP's blessing. He held
the worlds remaining supply of P channel FETs for fixing probes.
He was in New Jersey, but my contact info for him is no longer
valid.

I don't know how to identify them, but in principle an ohmmeter might
be able to identify the polarity of the FETs.

While we are on the topic, it turns out that the market is such
that a 4193A with probe might sell for $5000, but if you break
up the set, the probe is worth $4999 and the instrument is worth
$1. Well, maybe I exaggerated that a little, but not much :-)
I actually know someone who paid $5000 for just a 4193 probe.
He made the mistake of purchasing an instrument without a probe,
and paid way too much.

It is also worth noting that now you can buy a very nice vector
impedance meter from Tomco in Austrailia for only $3000 new.
I A/B'ed one of these with an HP one, and I will have to admit
that the Tomco is the real deal, not a cheap knockoff.

Rick Karlquist N6RK