Does anyone have a chart they are willing to share with the meaning of the code letters on the heads of Cat branded bolts? These are c. 1950's, all off of a 7U D4.

The ones pictured below have different letters (left to right in the picture are N K R C and L) opposite the CAT logo.

The letters are not related to size - the same letter will appear on several different size bolts.

well on normal high grade bolts an M refers to metric as to what it means in cat circles i dont know.

standard high grade bolts like those supplied by ajax fastners, bremick or john glen: grade 8 bolts will have 8 dashes or dots on the head or nut grade 5 will have 5 dashes or none at all.
if you look up the part nos for bolts as listed in the parts book for your machine you can find the current equivalent of that part and the specs for it. remember current part nos have 6 characters so add a zero after the letter to make up to six characters example 7F045 becomes 7F0045.

when looking for parts a google search can be very helpful i was able to find sites listing parts by national stock number which is a us govt part numbering system many cat items have NSN as they were supplied to gov depts like the core of engineers and dept transportation the sites also listed the superseded manufacturer pn and the current one it also lists specifications rather helpful when i had to get a gear made as i then had the pitch the gear was made to.

Hi Team,
here in Melbourne Australia bolts/fasteners from Cat Of Australia had Cat and an M opposite them--these were made locally by Ajax here in Melbourne for C of A.
Alas, with the closure of the Ford and GM Holden car plants Ajax is no more.

Have seen Cat with other letters on machines from the USA which could have been one of several different letters that to me indicate the place/plant of manufacture this would allow Cat to likely follow any warranty or production issues so they could be withdrawn from stock if required.

Below are a random selection of markings I have seen and saved since 1962 in my 3/8" UNC bolt box.
The local Ajax made Cat M stamped bolt head is on the right of the picture.

From memory I believe there is a Service Magazine or other article telling of Cat only supplying grade 8 bolts from that date on due to low grade bolts, that they used for floor plates and the like, being used in high stress locations and causing failures--when time allows I shall see if I can hunt it out.
Cheers,
Eddie B.