This is NOT accidental and it was so characteristic of this type of paper used then, that I tried to find an adequate description for it! The term pockmarked seemed to me rather appropriate:
Pockmarked or in Spanish "variolosa"
1. pockmarked marked by or as if by smallpox or acne or other eruptive skin disease;
2. pockmarked used of paved surfaces having holes or pits



to be continued ...