That 'JL' is familiar and has been found on a number of Belgian doubles.
The subject gun; possibly Laminated Steel but possibly there is a section of barrel where the contrast is more apparent?
Crowned 'JL' with crolle - likely pre-WWI Belgian double
Belgian unknown maker or DOM but 'JL' with ACO for Acier Cockerill so fluid steel
'JL' may be H.Jonlet & M. Lovet, but I can't find anything about them.
At least some Lifters have Birmingham proof marks on the tubes but we know "rough forged tubes" were also sourced from Belgium.