You are correct. No letters or numbers anywhere. This link has a bit of history about them.
FP-45 Liberator - Wikipedia
I had two, bought them in 1986 for $800 each. Decided to shoot one to see how it felt. Stove piped bullets. On shot 18 all the spot welds broke. I had a bags of parts. Sold the bag for $400 and have left this one alone since.