National Federation/Boys Varsity.
About one minute or so into the fourth quarter, at an "out of bounds" dead ball situation the "horn" blows!
The scorer informs us (the officials) that substitutes B6 and B7 who are playing in the game did not report to the table between quarters as substitutes starting the fourth quarter.
What is the penalty...if any for this situation?

No penalty. Too late.
Let me guess, B was the visiting team.

assuming they are entered in the book properly, once play starts (clock has started) they are now legal. No penalty.

Too late, they are legal players.

B was the visiting team, of course, and we both said that once they were in...they were in without penalty. Too late to penalize, not a corectable error. Team "A" wants a technical foul, don't they always. But we continued on from that point without incidence.

That's right.