Picked up this A. B. Co. wrench recently. It's the fourth wrench listed in Rathbone's first book with no indication of reverse side markings. Mine is marked "930 - FOR SLACK ADJUSTMER" on the reverse side. It's 11.5" long and 4" wide at the widest part.

Anybody have any info on the A. B. Co.?

 There was an American Brake Co and a slack adjuster is used to adjust air brakes. Found nothing definitive but wondering if it was for locomotive brake adjustment.

Jim,
I think you are correct. I found a catalog with parts lists for the American Brake Co, couldn't find a wrench but slack adjusters were listed. Thanks

http://books.google.com/books?id=P0FKAAAAYAAJ&pg=PP11&dq=%22American+Brake+Co%22&hl=en&sa=X&ei=T9AcUvGKDOm9sQSCj4HgCQ&ved=0CEYQ6AEwAA#v=onepage&q=wrench&f=false

I was involved for a short time, with the Kelsey Hayes Automatic Slack Adjuster program back in the early 80's. I found this with a search. At that time, SAAB was a brand that we competed with.

ge-reprints.com/catalog/1957-bromsregulator-svenska-ab-slack-adjusters-234406.html

I suspect Jim is correct, but will throw in Altorfer Bros Co,(Peoria,Il) which made washing machines (Farm Impl Buyers Guide V30,P315) , seems to have liked 'A-B-C" as a brand name tho...

How early would auto brake parts have to be to be square bolted? Any early tractors carry an A.B. name?