Yes, it is the adjuster. Marked "Fast" at the top (partially obscured) and "Slow" below to adjust the time-keeping.

It has been checked over by Max Schweizer, the Australian Rolex service and sales agent at the time, when he visited me in Darwin and his advice was to send it in to the Sydney shop for a precautionary service if I intended keeping it wound indefinitely, otherwise everything was in perfect order for winding it occasionally to show friends and fellow enthusiasts!

After perhaps a century or more, how good is that!