Which is it? I have seen it posted both ways, sometimes in the same post.

Here you go, thorough discussion

https://community.fantasyflightgames.com/index.php?/topic/178409-psa-its-xi7-not-x17/

X I 7

X i 7

There is no 1 in there

I just can't help saying X-seventeen instead of eleven-seven. Mostly because it really seems to upset some folks, I'm a jerk like that.