Hi Simon,
Your assumptions were almost exactly right:
Your composite arquebus consists of an early South German (almost certainly Nuremberg made) Landsknecht arquebus barrel; the way the barrel is staged - the first, rear stage seems to be round - assigns it to the 1520's-30's. The original long barrel tang and rear sight have been removed, and the pan is of Italian ca. 1570's type. The lock and stock are quite crude modern reconstructions and do not meet the correct style: while the lock is of incorrect 1630's style the stock vaguely recalls 1580's Italian stocks. As the stock is new the lock cannot be a replacement.
If you are interested in original Landsknecht arquebuses of ca. 1520-25, 1539 and ca. 1540 please see my threads
http://www.vikingsword.com/vb/showth...necht+arquebus
http://www.vikingsword.com/vb/showth...echt+harquebus
http://www.vikingsword.com/vb/showth...echt+harquebus
Best,
Michael