although nobody, except myself in my mind, asked this matter here, I like to share the result of the digging, because this Great Forum gave me all the fragments I needed, and since I had quite a job to gather all the info about mapfilenames for zumo 550, my ancient gps-compass with wildly winding needle...
I humbly hope this gives help for some other lonely rider with multiple maps...
ZUMO 550 inventory:
Suppmap = gmapsupp.img
Suppmap1 = gmapsupp.img
Suppmap = gmapsup2.img
PROM = gmapprom.img
Dir DB = gmapoem.img (not selectable...)