Okay, I just got through numbering my Arctic HISS Tank. I wasn't feeling all that imaginative, so I went with "482", which is shown on the package illustration, and besides, that hasn't been used on a HISS Tank before.
Interestingly, the instructions show the number "724", although the instructions also state "Any set of numbers". Of course given that you get two numbers of each 0-9, it had better be any set of three different numbers, really.
(Here's an odd thought -- there's plenty of room on the side of the tank. Why not four numbers? "1085", for example, would be perfectly possible).
I found myself wondering if "724" had ever been used before. I checked the Target exclusive blue HISS Tank that came out a while back -- nope, it's "827". And of course the original HISS Tank was "788".
Now, I didn't get the black HISS Tank that was released as part of the 25th-style line, and I honestly don't recall -- was it "788", or maybe did it use "724", or is Hasbro just showing possible variety here between the instructions and the box picture for the Arctic HISS? If "724" has never been used, that'd be cool for a second Arctic HISS.
I gotta get me some more of these Arctic HISS Tanks somehow. This is fun!