This is the World War I draft registration for Henry Brock, the sister of Naomi (Brock) Rosenthal [Harold's uncle].
Here's a copy of the (modern) transcription copied from ancestry.ca
U.S., World War I Draft Registration Cards, 1917-1918
Name: Henry Brock
City: Brooklyn
County: Kings
State: New York
Birth Date: 30 Sep 1884
Race: White
Draft Board: 083
Here are my notes about this document:
The third and final World War I draft registration in the U.S. took place only a couple months before the end of the war; it required men as young as 18 and as old as 45 to register.
The address (270 Saratoga Ave. in Brooklyn) is the same as the one in the 1915 census record that I think is for Henry and Leah Brock. See this address today in Google Maps.
Gives Henry's date of birth as Sept. 30, 1884.
He is naturalized.
For his present occupation, it might say "Emb Business" (i.e. Embroidery?); this matches his occupation in the 1920 census; so this must be the correct Henry.
The employer appears to say World Art Embroidery Works, 114 5th Ave., New York. See this address today in Google Maps.
His nearest relative is "Mrs. H. Brock" -- presumably this is Leah (Rosenthal) Brock.