This is not a Singer 111W, it is a Consew 226R, but they are almost identical machines. Are these the buttons you're referring to? If so, the one on the left is to change stitch length, and the one on the right is to re-engage the safety clutch on the main shaft under the bed. The safety clutch disengages when you jam the machine. It prevents further damage to smaller more sensitive parts.
BTW, the manual for the 111W152, 153, 154, and 155 will work for the 151, there are only minor differences between all of them.
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