What is "depicted" in the ad is a modified M91 Carcano (the original Carcano model) shortened from its 50.6" factory length to about 41".
What was described is a 36" Cacano that weighs 5.5 lbs. Like the model in the illustration, it also was a type of Carcano the company wasn't shipping. What they were shipping were 40" Carcano, which ads following this one described as that length (and a few ads later on, the weight was corrected).
I don't think that these cheap Italian Carcanos were much of a priority for Kleins considering the profit margin and they probably made more money on the scopes and ammo.
The following ads were both from April 1963 gun magazines and both show the same serial number C20-T750 but have differing weights and lengths but realistically from Kleins POV both rifles took the same ammo and the 40 inch was bonus so what's the problem?, it was a win win situation! It's not like a pair of shoes which must be an exact size, in the Carcano 6.5mm World it was one size fits all.
It seems that Oswald made his paper bag assuming that he received the 36 inch rifle, the look on Oswald's face when he realized his 38 inch bag wouldn't fit his 40 inch rifle would have been priceless! Hahaha!
JohnM