[Disestablishmentarian]

>>When/where was D5 decided to refer to avalanche?<<

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97% of anon scientists ...

Seriously, count me as one who is not convinced. I can easily imagine D5 as having a meaning that is not yet disclosed ... and that a Future INDISPUTABLE meaning (such as prominently featuring the term in the 20 pages) will be another proof of Q, i.e., Future proves Past.

[Duchess47]

I agree with you. I can see it referring to a chess move, chess had been referenced previously.

Trident missile is more of a stretch but missiles had been referenced.

I don’t recall an avalanche ever being referenced.

Meaning as yet unknown also works for me.

[Pearls Before Swine]

I like the avalanche, but if you want another reference, here's one for the Caterpillar D5: