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Bill I have an SA bolt with drawing number 7790186 on the top line, SA to the bottom left and then Z3 about a quarter inch to the right of that. What does the Z3 stand for. I have ZB marked on both my TRW bolts. I thought the SA bolts were marked with CA1?
 

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That is a code for what is called the "heat lot". It is industry jargon for keeping track of batches of steel. "Heat lot", as it is referred to, signifies which batch of steel from which steel supplier. SA and others had many lots of steel from each supplier and many suppliers of steel. As years passed new suppliers were used and old ones were dropped, depending upon their delivery terms, quality and price. I don't think we will ever find all the possible heat lot codes.
 
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