I acquired these modules from a source that was given to me by Philbrick enthusiast and frequent PhilbrickArchive contributor Achim Dassow. Dan Sheingold contributes some further info on these particular K3 modules in the following email.
Date: Fri, 2 Dec 2011 15:33:26 -0500
To: Joe Sousa <k2w at philbrickarchive dot org>
From: Daniel Sheingold
Subject: Re: My GAP experience
These are 2nd-generation K3s with fabricated cases and plugin K2 amplifiers. The first generation had cast-aluminum cases and integrated (on the board) amplifier wiring. The unidentified box at left was very likely a K3-J Integrator; note the upper input, which was a clamp to reset initial conditions. The clamp switch across the integrating capacitor was probably a neon lamp wrapped in a coil. When RF (1 MHz?) was applied, the neon would act as a bilateral switch and conduct like a FET. (My recollection is that tube triode switches had too much capacitance; the reset pulse would dump charge into the capacitor, establishing a faux initial condition.)