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I was getting a bit overenthusiastic about posting when new revelations hit.
The final email of that series is the executive summary:
? ??/g/BITX20/message/40918

So I concluded that the Bitx40's T7 *is* a flyback transformer, and your 2n7000 could see kilovolts
if the antenna port is left open circuit.? (In this case an autotransformer, part of it is both primary and secondary)
But only if the RF is of fairly low frequency, and on the Bitx40 it never will be low enough in freq to care.

At 7mhz, the IRF510 never shuts off, even if I drive the gate directly from a square wave LTSpice voltage source.
Probably because as drain current decreases, the drain voltage shoots up due to T7 wanting to keep the current going,
and the miller capacitance causes the gate (internal, not necessarily the TO220 pin)? to rise as well,
tending to keep the IRF510 turned on.? Negative feedback at work.

As the Wikipedia entry shows, a flyback converter has a diode on the secondary.

I've worked around buck and boost switching converters, and had to figure out what those inductors and diodes are doing.
Moving to a flyback transformer is not much of a transition, it just adds another winding that can tap into
the same energy that the first one sees.? ?Which is to say that in addition to your 2n7000, the IRF510 could also blow
if the antenna port is left open at low frequencies.? Which explains the zener from drain to ground
you often see in this kind of amp.

The uBitx probably won't have this issue, I believe T11 truly does behaves as a forward transformer.

Jerry, KE7ER