Origin of term 'french letter'?

I would like to know what the origin is of the word French letter (for: condom).

The use of the term 'letter' seems quite straightforward: a condom is a bit like an envelope in that it envelopes ...

The french have a reputation of being romantic.

All this is speculation. I found one place on the Net where it says it dates back to 1856. No more.

Corine

The first contaceptive devices of this type were developed in France. They were made of animal organs - possibly intestine and were later imported into Britain They became known there by their present name. All of this in the early 1800s, I think.