Joseph…………..
Many of their publications were not copyrighted by HPC. Some were simply typeset by them copying siddurim from Europe.
US Copyrights expire.
All copyrights for things published before 1923 are expired. Things published before 1978 had a 28 year copyright if not renewed. Since 1978 new copyrights generally have 95 years from publication or 120 years from creation (whichever is shorter).
Sadly, HPC is defunct as a publisher, but some individual may own the name.