The rabbit hole is deep, and confusing
We've found these marks on U.S. doubles, and we have good evidence the marks
do NOT mean Sanderson Bros. made the tubesS.B. & Co - Sanderson Brothers & Co.; follow by
G – Gautier?
In 1882, Gautier Steel was a division of Cambria Iron Co., Allegheny Co., PA. Dudley G. Gautier was president of the Philadelphia Steel & Forging Co. and D.G. Gautier & Co. was listed as an agent for both Sanderson Brothers & Co. and Tacony Iron Works (1881-1910) of Philadelphia.
T – Tacony?
P – Sanderson‘s Pittsburgh Works?
C – Sanderson‘s Cumberland (Maryland) Works? or Carpenter Steel Company, establish in 1889 in Reading, PA which used a ‘C’ to mark their bar steel?
Here is an Ithaca Flues with the same mark
Long version is here
https://docs.google.com/document/pub?id=17ixogftgITEblNUWtmFBv96ZvgjK6eFell8GsAWd-KI