Necchi 537-L Sandi, Machine made in Taiwan, My guess on age would be early 80's. Links below may help, first link one lady states buy her 537L from sears in 1982., other links are instruction and info.
Check the attached links,instruction and guides, Good luck
"I hope this helped you out, if so let me know by pressing the helpful button. Check out some of my other posts if you need more tips and info."

Life With Lou Necchi 537L Sewing Machine

Necchi 536 FB 537 FA Sewing Machine Threading Diagram

http://www.sewingpartsonline.com/necchi-sewing-machine-instruction-manuals.aspx

Try google ---www.sewing-machines.blogspot.com.au as there is information as to the history of necchi machines
they were originally made in Italy to exacting standards and in 1950 an zigzag model was introduced
starting with the lydia model the machine started using plastic in the cam and other components so if yours has plastic inside, it is after 1950
there is another site-- www.geocities.com/lamx --that has a necchi collection so you may get information from there as well
I found sites after typing in --year of manufacture of necchi 537l sewing machine --so if you navigate around all the available sites you may get what you are after