Is there such a thing as an "HS" stamp for a 309 brake master cylinder
 

I found a 309 brake master cylinder in my garage and the stamp is pitted. The "S" is clear but the first letter is either a "U" or an "H" but it is hard to read because of the pitting. I have been wrong before on my parts so I thought I would ask before I advertise it.

Check the Julian date "US" stamp would be a 69 Camaro
starting in the fall of 68

Most likely a U.

I agree with the other members that it would be a US stamp. I have one very similar with pitting and its hard to see the "U" of the US stamp. Mine is dated 274. The only other stampings for a 69 5468309 master cylinder that I know of would be the following:

69 Camaro & Nova US Stamp
69 GTO EA Stamp
69 Chevelle BA Stamp
69 Firebird SA Stamp

Ive seen the 5460346 Masters that look similar with ZA Stamp but not sure on those and apps.