How was Infocom's username functionality discovered?

How was it discovered that Infocom built into the Z-machine the ability to discern the username of who was playing the game? This was discovered long before the Infocom source code became available, as can be seen in the source code of very old versions of Frotz. In the comments (not written by me) is (optional; not used by any game) beside this. I can’t seem to find reference to this in the Z-machine standard documents.

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Look at Hex 38 in the first table at The Z-Machine Standards Document

The question is how did people figure that out.

Early Z-code analysis worked by disassembling Infocom’s shipped interpreters. It’s possible that those interpreters supported that field, even though none of the shipped game files used it. I don’t know whether that’s true, though.

Hollywood Hijinx uses the field, as discovered by Torbjörn Andersson in his ZIL source archaeology: Debug code in Infocom's Hollywood Hijinx

Ah, right, I forgot about that case.

What I meant to say is “no shipped game files have nonzero values in that field.” But since it’s an interpreter-filled field, that’s always going to be true! I think.

Anyway, the Hijinx code is a better guess from where that info game from.