Good news...
I was home this morning waiting to take my cat to a veterinary appointment and the snail drum package arrived. I had a quick chance to see how it fits in my shooters and it fits great (goes all the way in with an audible click when the mag release locks) in both my 1918 DWM shooter and my 1918 Erfurt frame that's been paired with a 1917 DWM receiver and artillery barrel.
The gun that doesn't want to readily accept the drum mag is my Alphabet commercial .30 cal (k suffix). It barely starts into the frame before it binds, so using the drum with .30 cal ammo may not be an option (for me) after all.
I have a number of other non-shooting Lugers, both 7.65 and 9mm, that I will try the drum in and report back about fitment.
Hopefully I'll have time tonight to configure the loading strut and learn how to use the loader.
I was surprised at how heavy the drum is, by the way. A very solid unit to say the least.