One of my science LORs (i.e, certainly not an "additional" letter by any means) was not on a letterhead - I found out because two schools (out of over 20 that I applied to) contacted me to let me know they could not accept the letter without letterhead. All other schools seemingly had no issues with it - I was offered many interviews and was accepted everywhere I interviewed. Worth noting, at my top choice school, I only submitted two letters - the un-letterheaded science one and my likely average DO letter. Still was accepted, so can't imagine this was actually a big deal. YMMV but certainly, don't panic, especially if you can't get in contact with your letter writer.