Hi Susi and welcome here,
Thank you for your interesting input from the Zeitschrift für historische Waffen- und Kostümkunde
. I have been a member of their society for 30 years and own all their volumes from 1897 thru 2010.
First, these are by no means exchangebale breeches for 'handguns' but for cannon
! Portable handguns of the 15th century had small bores of ca. 12-16 mm and their breech chamber never had handles; they had no grips at all and and an overall length of only ca. 6-8 cm. Please cf. the measurements of the 1540 Passau arquebus breech chamber you cited!
Second, these seem to be exceptions to the rule. For these unregulated ages, almost anything was possible in general. As I noted though earlier in this thread, we know of other loaded breech loading cannon chambers only containing powder and wooden plug, and attached I repost my own photos of one of them. The rest of the original handle can bee seen.
Similar ones were found on the wreck of the Mary Rose, sunken in 1545, also just loaded with powder and plug.
'Nando, my dear friend, would you mind posting good images and measurements of your ca. 1440 fine cannon chamber as this is almost identical to the ones preserved in Berlin?