There is no MS sold in the USA. US Medium = ML or M on size chart.

Inches 20 1/8 20 1/2 20 7/8 21 1/4 21 5/8 22 22 3/8 22 3/4 23 1/8 23 1/2 24 24 3/8 24 3/4 25 1/8
CM 51 52 53 54 55 56 57 58 59 60 61 62 63 64
Hat Size 6 3/8 6 1/2 6 5/8 6 3/4 6 7/8 7 7 1/8 7 1/4 7 3/8 7 1/2 7 5/8 7 3/4 7 7/8 8
TITECH (for Easy Fit) XS XS S S MS MS ML L XL XL XXL XXL XXL  
TITECH (for Race Fit) XXS XXS XS XS S S MS ML L L XL XL XXL XXL
XR2 XXS XXS XS XS S S MS ML L L XL XL XXL XXL
STEALTH XXS XXS XS XS S S M M L L XL XL XXL XXL
V-FLYER XXS XXS XS XS S S MS ML L L XL XL XXL XXL
DAYSTAR XXS XXS XS XS S S MS ML L L XL XL XXL XXL
MIGLIA XXS XXS XS XS S S M M L L XL XL XXL XXL
GT OPEN XXS XXS XS XS S S M M L L XL XL XXL XXL
THUNDER XXS XXS XS XS S S M M L L XL XL XXL XXL
DRAGON XXS XXS XS XS S S M M L L XL XL XXL XXL
A4 XXS XXS XS XS S S M M L L XL XL XXL XXL
RC5 XXS XXS XS XS S S M M L L XL XL XXL XXL
AX-6 XXS XXS XS XS S S M M L L XL XL XXL XXL

AX-7

 XXS XXS XS XS S S M M L L XL XL XXL XXL