I just found the Singer parts chart for your machine. The part in question is # 514121 the "upper feed regulating stud" which means my guess was right, that's how you adjust the machine's needle feed.
My advice is that if it's sewing O.K. right now, don't ever change it.
Here's a direct link to the instruction manual: http://parts.singerco.com/IPsvcManua...66_265_266.pdf
I couldn't find any mention of that button in the manual.
No one lives forever, the trick is creating something that will.
Last edited by DanTwoLakes; 02-07-2013 at 08:38 AM.