The design side at least seems good...in my opinion from the pics, they did a very good job of using an odd shape efficiently. I am tempted, as a random hoarder with Nazi, WWII USA-USMC-British cans, and a few modern euro type cans stashed around.
I just found surplus US military adaptors that mate a standard US fuel spout to European type cans...don't know if it was made to interface USMC cans with other US or for interchange with British and Wehrmacht cans.
A period use comes to mind from something I heard from a guy who went to college in the '50's and '60's and hitchiked home weekends and holidays: He found that drivers would readily pick up people standing by the road with a gas can, but were suspicious of a guy wth a suitcase or pack...so he cut a hatch into a gas can and carried his toothbrush and clean socks that way!