Try this it should work
(defun c:test ( / int sel ent get)
(setq int -1)
(setq sel (ssget '((0 . "INSERT") (2 . "BLK-01"))));BLK-01
(while (setq int (1+ int)
ent (ssname sel int)
);setq
(while (/= (cdr (assoc 0 (setq get (entget (setq ent (entnext ent)))))) "SEQEND")
(if (= (cdr (assoc 2 get)) "ATT-01");ATT-01
(entupd (cdr (assoc -1 (entmod (subst (cons 41 0.75) (assoc 41 get) get)))));0.75
);if
);while
);while
);defun
edit: localised all variables