[Raf]

The lock looks Italian and I agree with you that it could have started life as a late 16 th century petronel. It looks to me like it's been re stocked , possibly mid seventeenth century using a stock blank intended for an Italian wheelock. The rebate you point out on the top of the barrel could be the original location of the back site which on early matchlocks tends to be at the breech end of the barrel. The style of the back site looks more mid seventeenth century and could have been added when it was re stocked. Or as you say this might be the remains of attachment of a 'tube sight'
Your reference to tube sights is interesting . The popular view, shared by 'Matchlock' on this forum is that they are indeed tube sights but I'm not sure I agree. Robert Held, The age of firearms 1959 is adamant that these are match guides intended to help manage the match cord by keeping it away from the flash pan. He provides a drawing to illustrate. I will quote the reference here so that you can make up your own mind.

'The match cord itself was protected from the flash by being passed from the serpentine into a tube on the top of the barrel, out again behind the flash shielded then wound around the stcck. These match tube guides are often mistaken by museum visitors for tubular rear sights and indeed some of them may have been just that. Those which are closed either end or in the middle by an iron disk or diaphragm pierced by a small pinhole aperture were obviously tube sightes...those that were unobstructed were match guides.'