For $150 you got the deal of the century! That sold for about $1600 new and I worked 35-40 hours a week at a second job to pay for mine.
If that is shifting well don't mess with it. Keep it very clean and find a old mechanic who remembers when that bike was new, maybe he can fix it it is needs repair.
That smaller lever is to set the shifter to friction shifting should there be an issue with the rear derailleur, like getting bent in a crash or a wheel change with a team mate or something.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk