SP's (real and imagined) are looking like a combination or influence of:
- Composition or technical issues (pulling a row, subbing another real quick) prior to printing
- Number of impressions per card on the sheets
- Card coordinate on sheet
- Superstars being called SP's instead of "high demand"
- Collation and Packaging Damage
- Original Distribution skewing pops
- Boxes being opened in the old hobby days with bad collation
- After Market Events (1975 Card Collectors Co fire) skewing remaining inventories
Most years seem to follow a 4:3 or 3:2 ratio for overprints but some like 1967 seem to have an outlier row.