There certainly was a problem of inconsistent dimensions among manufacturers of older shutters. I didn't know when that was standardized.
I've done an awful lot of shutter repairs, most being for complete lenses, and I only recall finding shims in one lens.
Modern machining methods can produce parts (and complete shutters) to very tight dimensional tolerances.
In the days before CNC machine processes, making parts to tight tolerances cost a lot more than using lax tolerances.
In that situation it was much cheaper to make the shutter slightly short and use shims to achieve the correct dimension.
This also provided correction for dimensional errors in the cells themselves, if the lens was set up on an optical bench.