Isn't that exactly what MTS ("Member of Technical Staff") and "slotting" were at google, at least up until around 2012? They hired people not knowing exactly where to level them, and after an introductory period, they were placed somewhere along L4 to L6.
I had to look this up because I'd never heard of it.
Slotting and MTS existed, but indeed hasn't been done since 2011 or 2012, and hadn't applied to L3 or L4 candidates for much longer, which means it likely was a way to differentiate between borderline L5 and L6 candidates or borderline L6 and L7.