Suzuki Book Two Notation
What do these symbols mean (circled in red)? I assume the arrow inside parentheses is an extension symbol and the bracketed numbers are probably optional finger positions, indicating a shift instead of an extension, but I'm not certain and couldn't find anything online.
Suzuki Cello School Volume 2, Piece 9, Theme From \"Witches' Dance\"
Related to this, I'm unable to stretch my hand far enough for the extension while keeping the rest of my fingers in 1st position despite having fairly large hands. This is probably because I'm an adult learner and have never done an activity that stretches my fingers apart before (current unassisted flat spread of my hand from tip of pinky to tip of pointer finger is 7 inches). Should I practice the extension or just shift? I feel like I will be stuck on this piece for a long while if I try to play it with the extension.