The Tamron SP 17 mm f/ 3.5 Adaptall-2 model 151B is an ultra-wide angle prime lens for 35 mm SLR cameras popular in the 70-90s (via the interchangeable Adaptall-2 mount).
SP (Super Performance) - the lineup of Tamron best lenses.
This lens has replaced Model 51B. The changes have affected both the body design and the lens construction (no built-in filters for black and white photography, filter thread available). Despite replacing some body elements with plastic, the build is still excellent, though inferior to the early version.