The sump filter is cleanable, if you find it torn a $5 package of JB Weld can fix the hole. Even Mike's reinforced sump filters will tear. The A,B,C models represent the years. The 70 was an XS1, the 71 an XS2, 73 was a TX 650, 74 was a TX650 A In 75 they went back to the XS prefix but kept the model suffix in order, as in 75 was the XS650 B. 76 was the XS650 C, 77 the XS650 D and so on. In 78 the added the Specials So they added the S in the model name, as in the 78 standard was XS650 E the special was the
XS650 SE, they had a Special II for a few years and they added a 2 at the end to show them. As in 79 XS650 SF2.
Leo