According to Farnsworth, when Debosnys died his body was found to be covered with shocking tattoos. This was not an uncommon practice among criminals in 19th century France and Italy, and the criminal tattoos I have found are strongly reminiscent of the style of art in Debosnys' manuscripts.
For example, Debosnys drew this handshake, which Farnsworth identifies as a Masonic grip called Boaz:
But this is also a common motif on 19th century criminal tattoos, according to Lombroso, who presents some examples in his L'uomo delinquente, such as this simple one:
The following example is reminiscent of the ritual of blood-brotherhood, where the hands are cut and the cuts are pressed together to symbolically join two people by blood.
This one looks like it might symbolize the union of two people, LH and EL. The flower suggests a romantic relationship, but maybe something else.
Lombroso says this one shows a preference for pederasty, taken in context with other tattoos on the prisoner's body:
Whatever the specific meaning of a handshake, in general it symbolizes some kind of close connection between two people. In this case, between Debosnys and LMF.