Answer:
The PN521 has a triple-corrugated shell, protective peak, ratchet-type adjustment, adjustable chin strap and side slots for face and hearing protection. ERP also notes an extended nape.
KARAM lists a triple-corrugated shell, protective peak, ratchet-type adjustment and adjustable chin strap for the PN521. The product page says its slide slots allow attachment of face and hearing protection accessories. ERP also describes an extended nape and polypropylene co-polymer shell.
