Go online and google free manuals. Penney machines were either Singer or Brother; I do believe.
Okay to change a zigzag to a straight stitch, put the stitch width to 0...that way the machine goes zero degrees to the left or right for a zigzag. The stitch lenght will change that for your straight stitch also. I do think that it will line up with the lenght for straight stitch measurements also.
You can google the machine model and see what comes up for years. I found the year; the country and so on for the treadle machine I have...I seem to collect them. Good luck and keep sewing.