In an effort to solve the oscillation issue in his first prototype and which you noted, one thing Roger found is Futterman himself omitted some things from the H3 schematic, specifically the lack of notation for the ferrite beads@clio09
IMO this was intentional. I have a lot of anecdotal evidence from customers that say that the Futterman amps held together while amps made by others using the Futterman circuit didn't. However Harry Pearson recounted an incident where Futterman brought one of his amps to Sea Cliff for audition but before it could be entirely set up, one of the amps went into oscillation and failed (this was in response to a letter to the editor from Harvey Rosenburg in the late 1990s). But all you have to do is leave out one bead by accident and you're sunk. Futterman made most of his amps himself to my understanding.
60dB of feedback in a tube amplifier is an impressive feat! Normally you have such prodigious issues with not exceeding the phase margin of the amp (OTL or not) that most would not attempt such a thing. If 60dB is correct I'm quite impressed (and stand corrected)! The ones I've seen did not have any such value- I doubt that they even had 60dB of loop gain (that's the gain of the amp plus the amount of feedback). But some of the Futterman amps have impressively low output impedance figures, such that they would have easily behaved as a voltage source, even though they made more power into higher impedances.
Kron-Hite made laboratory amplifiers in the 1960s. In their manual for the amp (which used KT88s) they claimed 80dB(!) of feedback. Its hard to imagine how they pulled that off- that amp was quite stable. I had a pair of them for a while in the late 1970s and they compared very favorably to an ARC D-75 that a friend of mine had.