The part you show in third picture in Post #10 with the "R" is commonly called the "cocking piece". It is attached to the rear of the firing pin, slides within the bolt plug / bolt sleeve, has a face that is acted on by the trigger sear and has a notch on the side to be acted on by the safety. Apparently depends whether you are Britishicon or American for the correct name, but the "bolt plug a.k.a. bolt sleeve" or "bolt shroud" (if it was a Mauser rifle) screws into the rear of the bolt body. So, you have an "E" bolt body with an "R" cocking piece. The firing pin and the "bolt plug/sleeve" will also have a maker's mark.

A quick and easy reference for M1917 parts markings can be found on m1903.com > Marks > M1917 parts. Some minor errors, but very handy reference. Click on the part name and a sketch pops up showing where each maker put their mark on a part - sometimes the same place, most times not.

Besides the above mentioned book by C. S. Ferris, consider getting the book "Pattern 1914 and U.S. Model of 1917" by Charles R. Stratton.

The original US Army document - something like "Rules Of Management for US Rifle of 1917" is readily available on Internet and shows procedures that WWI soldiers were taught to disassemble and re-assemble. They would have been able to disassemble and reassemble the bolt assembly while sitting in the rain in a muddy ditch, without using any tools! Also good information found on the CMPicon site about these rifles.