Can old employment reference letter be used as experience letter for Green Card application?

#1
My current employer (I am holding a H-1B visa) is sponsoring my Green Card application. I have a dilemma: a small company I worked for more than 10 years ago has closed down. I am not able to contact any of my former colleagues from there. I have, however, a reference letter from my former supervisor from that company when I left the job. Can I use that letter in lieu of an experience letter for that job? Can someone advise me if there was similar example and whether it worked or not? Thank you very much.
 
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