A very quick G##gle search threw up this:
https://www.justanswer.com/general/793oe-merritt-39a3-breaks-needles-when-back-stitch-feed.html
which then led here:
http://parts.singerco.com/IPsvcManuals/391A1_3.pdf
Did that help? Please post back and let us know, August 2019