you should of saved a weapon token... ??
Well anyway on youe next rank '40' you unlock one weapon token, which you can then use on the weapons unlocked at Lvl 39, so, M20 Gauss, X-43 Mike and the C4.
So if you had saved up some weapon tokens befor Lvl 39 you would have been able to get the weapon you wanted.
for the ranks 40, 43 and 45 you will get weapon tokens.
I hope this helped.