[ARC5] BC-221-xx v. LM-xx Questions

[J Forster]

I've just been playing with a BC-221-AK and an LM-18 Frequency Meter and
have a couple of questions:

First, what is the family tree of the units? Specifically I was
surprised at how early the LM-18 design and components (especially the
tubes) appeared compared to the BC-221. Was this Navy conservatism or
what? Was the BC-221 a re-engineered LM-xx?

Second, are there any modern measurements comparing frequency stability
and temperature coefficient of the two designs? I'm not interested in
design specs, but rather measured data.

Thanks,
-John