The 2026-2027 DO School Specific Threads are now available in the School Specific Discussions forum. The 2025-2026 discussions are now available in the prior year discussions forum.
There's not way to know whether your writer signed or not through interfolio. You have to ask the writer personally.
Officially letter head is required at 99% of ther schools. It just means that the lor must be printed on offical university paper or offical clinic or hospital papers. ( with the logos and address/department/ blah blah on somewhere of the paper) I believe this process adds credibility, since it's hard for students to obtain or steal such papers!