Thanks SeanValli, I tried the same or similar - saving an appointment using "Save As". Then do the lock/unlock cycle, and again "Save As". The resulting second file is different but from what I can tell it will always be different because it saves creation/modification timestamps inside the calender item.
For example when saving as .ics format, the file is plain text and the ics fields CREATED, DTSTAMP, and LAST-MODIFIED differ. But this is a result of saving the file at a different time and is to be expected.
When saving as binary .msg format I suspect that such fields are also included in a binary encoding so it becomes difficult to see what is really changed.
So it's clear that the lock/unlock cycle changes the open appointment in some way, but we can't see yet what is really changed or why.