So this is the best answer I can come up with. Hopefully someone can expand on it and/or correct it.
There are 4 basic pass blocking scheme concepts. 50's, 60's, 70's and 80's.
Note: There are different identifiers for Run Blocking schemes.
So it's not an individual designation. e.g. 73 doesn't mean 7 linemen or blockers etc.... although even numbers seem to mean right and odd numbers left.
What each Pass blocking scheme identifies:
1. Whether vs Man or Zone and /or vs 3/4 or 4/3 Defensive fronts.
2. Who and how many are responsible for pass protection.
3. Which Defender each player is responsible for.
4. Which Offensive player is the Hot read.
5. The QB's drop. (Under center, Shotgun, 3,5,7 step drops)
6. Who the QB should identify as the Mike.
7. QB Audibles.
8. And a few other things.
So back to your (and my) question of what does 73 mean.
It means: (I think)
1. The QB is under center and I believe a 5 step drop.
2. Only the Offensive linemen are pass protecting. RB and TE are releasing into passing routes.
3. One of the Receivers/RB/TE will be a Hot route.
4. And more that I'm not sure about.
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