The 3742 Smart Guide is shaped like an oval (rounded on both sides) while the 3741 is rounded on one side with a very slight arc on the other (please review the guide link in the intro).

If you wish to replace the entire unit because it cannot be made functional, the sub-version is either A or B, and you can tell because of the letter that appears after the numbers behind the battery compartment (Yellow Circle)

(If Replacement is needed, the type and version matters for which Oral-B Toothbrush you have)

The cover is held on with 2 clips (One on each side)(Red). It can be removed by inserting a screwdriver in the small gap (Orange) and pressing on the clip on one (or both) sides to pop out that side, then the entire inner assembly can be removed

The Board is held against the Battery terminals by 4 plastic clips in the assembly (teal), which can be easily pushed out of the way to remove the board.

Access to the circuit board contact pad and battery spring loaded contacts, which is often the culprit for issues (green circle). I cleaned off the corrosion with a knife/screwdriver, then did a final cleaning with 99% isopropyl alcohol and q-tips before reassembling and it started working again.

(Note that the discoloration might still be there, as seen in the picture, but if its clean, it will still function)