Every hex head bolt on my 1935 Willys Coupe has a number on the head. The same number appears on different sizes and threads. What is the significance of the number? Who made these bolts and for how long were they manufactured? Numbers are: 10,20,30,31, and 35.

FYI, here are pictures of the markings Bruce is curious about. What do the numbers mean?

Bruce, I wish I could answer your question. Auburns 34-36 also used numbered bolts on some applications. The oil pans were secured with 5/16 x 18 x 3/4 bolts marked 30. It seems that all of the bolt heads of the 30's era were taller than the modern varity.

I have also noticed that while looking for cadmium plated bolts for my Packard, that Harleys have bolt heads labeled 1038CP.

American Standards ASA B18.13 General Data 2.1.2.2-Identification
"...the manufacturer may add his identification to this area of the bolt head." Later on, they added hash marks to identify heat treated hardness.

C 1038 Steel Heat Treated (high carbon cap screws) are commonplace for head bolts.
So, 1038CP means it's Parkerized (A process of rust-proofing steel by immersion in a hot chemical solution which changes the surface of the steel to an insuluble phosphate-dark gray in color-black when stained.