If by master back set, you mean one specimen of each different back variety, the number should be around 40 (depending on what you include and exclude). A printable back checklist can be found at t206.org.
The third Texas league back would be hindu, assuming the first two are piedmont and old mill.
All the best,
David
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T206: 130/518 
T206 HZ: 6/6 
T206 SLers: 48/48 
T206 back run: 21/38 
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