Japan High Speed Agency Industry, Tokyo, 1953-60
The Japan High Speed Agency was set up in 1953 by Yamada Ring Sengokan (山田輪盛館) which was in business from 1909 through till 1968 (per the Japanese Wikipedia). The "Century of Japanese Motorcycles" also lists the company under the name "Yamarin" (ヤマリン) but I've only found references to a prototype racing 150cc model of that name patterned after an Ariel in 1951 by Yamada Ring, and which was the design basis for the commercially available 196cc ohc single HOSK BA. The subsidiary, Japan High Speed Agency, made the HOSK or Hosuku (ホスク号) brand of motorcycles which included both 2 stroke (123cc) and 4 stroke machines in displacements ranging from 196cc through to 500cc. The 500cc twin released in 1955 is interesting as it was supposedly a refined copy of the 1951 Horex 400cc twin called the 'Imperator'.
HOSK is an acronym based on the first initials of the design team (Hori, Ozeki, Shimizu and Kimura).
The production rights to the HOSK were sold to Showa in 1959 and then again in 1960 to Yamaha who went on to develop the XS650 from the HOSK design. The 1959 transition year produced models having the HOSK name, but using Showa frames, and possibly also Showa engines.