Seems to Me since It's a cable that engages the Overdrive gear a Ratchet Bar Lever will release to slack the Cable for Low /
and flipping the lever pulling a cable taught till it stops, to engage the overdrive , because of the ratchet lever Pawl will not let it slip back.
You're good to Go. Its not like You even have to find a Middle Gear..

I put together a S-A 3 speed, triple Cluster Cog set, and a Triple crank . I think I was 12, so thats about 1959.
I used 3 friction levers on my 3cubed 27 speed

Have Your Husband get another, Sun Race Taiwan is still Going strong..they own the former S-A and are making them .
better than the British did and expanded the product line substantially.