After taking a Model M under a magnifier this evening, I think the font is a closer to Helvetica than to AG Book, most clearly in "%" and "&". However, some characters don't really match either ("@", "~", and "g"). Also, the ends of strokes are all radiused, and both the large characters and small characters (labels) have the same radius of about .2mm, which means that the ends of strokes on the small characters are half-round, while the large characters only have rounded corners. That makes me think that perhaps it started with Helvetica or a clone, was converted to a form cut by a small round tool (e.g. for doubleshot molds), and then that form with its rounded corners was converted back for printing.