Jones 431 straight stitch

[Button Nose]

Help! I've been helping someone who has dug out an old Jones 451 sewing machine. We've got it threaded and going  . In the days between seeing it for the first time last week to now it has changed from straight stitch to zig zag. There is a button with M and a couple of zig zag/button hole options. It's on M but sewing zig zag. Does anyone have any experience of this machine and can point me in the right direction? I don't have the machine here so will have to relay any suggestions to the owner. 

[Gernella]

If you are on Facebook @Button Nose :

https://www.facebook.com/kchisewingmachines/photos/a.1665489507047915/2112959962300865/?type=3&theater

They have got it on the M position, have you tried the other positions to see what happens.  You can get a manual but would involve paying money.  I think sometimes you have to do a bit of experimenting on machines. 

[StitchinTime]

Is there a lever or dial that sets zig zag width? If so this should probably be on zero for straight stitch.

[Button Nose]

Ah, that rings a bell, Stitchin, thank you. I've got a brain like a sieve 

Thank you both