I have a 1903 Springfield Mark I with SN# 1063XXX. I am trying to figure out the markings on the bolt. On the lug in the middle of the bolt is NS which I understand to mean Nickel Steel. The mark that I can't figure out is UNDER the bolt handle. The ordnance bomb is there but under the bomb are two characters that I can’t figure out. They may be 50 or SO or 5D or SD, but they are so small I can’t tell. Any idea what they are? Also between the bolt handle and safety on the right side is a R. Any idea what these markings are? Who made the bolt and when? Any help will be appreciated.