Whites Spectrum XLT, Garrett AT Pro, Macro pinpointer, Garrett carrot pinpointer,
Lesch digger, Nel Tornado coil for ATPro, Garret ATMax with Nel Tornado coil
The range is 1902-1935. 1902 indicates when the Great Seal buttons started being used by the Army and many manufacturers (Waterbury, Scovill, Horstmann, et al.) made similar buttons. I believe Horstmann stopped making military buttons in 1935. Now the backstamp of "WH Horstmann & Co. (dot) Phila Pa (dot)" can be confusing as that type of backmark harkens back to Horstmann's civil war era but my understanding is they may have used old dies when making these "newer" buttons at the turn of the 20th century and thereafter.
Like many ID questions on more commonly found items, there is a TN thread that has been there done that, including this one.