Well, I don't know of an on-line source for thread specifications (I presume you can Google as well as I), but my Machinery's Handbook (24th Edition) has a table from ANSI B1.9-1973 on page 1631 showing specs for the standard Buttress thread sizes. 13 does not appear to be a standard Buttress thread, however the handbook provides formulas to calculate a custom thread profile. Are you sure this is a Buttress thread call-out and not something else? Does it also specify the direction of the thread profile? (Buttress thread is non-symmetric; if you form it "upside-down" the parts will not assemble.) I would recommend going back to the customer and asking for a clarification. Just because some engineer put a note or call-out on a drawing that he thinks is perfectly clear, does not make it so. It is far cheaper to ask for clarification than to make a wrong part. Good luck.
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You don't give enough info about your application but could BT be a thread cutting screw? Unfortunately, the 13 doesn't make any sense as the thread series runs from #12 to 1/4 like all American Standard threads.
Machinery's Handbook describes all the thread cutting/forming screw variations and info is available online. Google bt screw thread.
Buttress Threads 
Re: Details about Buttress thread
