GI bolts are identified by the drawing # 7790185 or 7790186 with the manufacturer and heat lot stamped under that number. GI lettering is specified to be exactly 1/16 of an inch high. Along with the usual manufacturers (TRW, SA, HRT) if your bolt is marked 66118 under the drawing # it is also USGI (Winchester). GI bolts may or may not have a dimple from a pin punch.
The real ID problem is telling real SA bolts apart from Springfield Inc M1A bolts as they also use the drawing number 7790186 and stamp SA as the manufacturer. Usually they use larger font but i have seen some with lettering that was 1/16. On real SA bolts the "SA" can appear either after the drawing # on the same line (usually 7790185 bolts) or under it in front of the heat lot (7790186 bolts).
The way to ID the Sproingfield bolts if by the letter prefix under the drawing #.
From the July 03, 2007 draft of the Third Edition of M14 Rifle History and Development:
Bolt - Springfield Armory, Inc. M1A bolts are manufactured in the United States of America. The M1A bolts are typically marked 7790186-SA on the first line and A00030, B00048 or F00059 or similar number on the second line. They may have markings such as D and M3 on the rear end and A9 or B1 on the bottom surface. The letter A prefix for the number under 7790186-SA means the bolt was cast then finish machined. The letter B prefix for the number under 7790186-SA means the bolt was machined from bar stock. The letter F prefix for the number under 7790186-SA means the bolt is forged. Springfield Armory, Inc. began installing F prefix M1A bolts by no later than 2000.
See 1987 Springfield Armory, Inc. Recall Notice for additional M1A bolt markings. No M1A parts, including bolts, are made by metal injection molding. Around receiver serial number 165###, Springfield Armory, Inc. commercial manufacture factory installed bolts have letters and numerals with a taller and thinner font than the style found on USGI M14 bolts. A Springfield Armory, Inc. M1A bolt not subject to recall was tested for surface hardness. The result was 52 HRC, the same as a USGI HRT marked M14 bolt.
USGI heat lot codes are usually 2-3 letters/numbers no more.
Left to right: Bolts 1-3 Real SA. Bolt 4- Sproingfield. Note that bolt 3 is 7790185.
