I have a theory on this, based on how LG pushed the G3 release sooner than previous years. The G2 was released in September 2013 and the G3 has already been released in Korea and is rumored for a July release in the US. That's only 10 months apart.
The G Pro 2 carried the model F350 and was released February 2014.
LG G Pro 2 - Full phone specifications
Maybe the F460L is a G Pro 3 and is being targeted for a December release, which would put it in a similar 10 month window (like the G2 to the G3).
And, what has been kinda swept under the rug is the fact that a few US carriers will be releasing the G Pro 2 lite
LG G pro 2 lite (LG-VS880) for Verizon Wireless Approved By The FCC, 5.7 inch display,3200 mAh battery
So, it all would make sense to me if LG were to offer a 5.5, 5.7, and 5.9 (G Pro 2 and G Pro 3), with the 5.5 G3 and 5.9 G Pro 3 being their offset flagships, similar to Samsung GS5 and Note 4 (of course, Samsung also has the Galaxy F coming up to add to all of this).
Considering what LG has done with the G3, I'm definitely paying close attention to what they do for a follow-up to the G Pro 2 (and if it's made available around the world, not just Korea). If they could make a G Pro 3 that was similar in size to the Note 3 (if not smaller) and still stick in a 5.9 or even 6.0 inch screen, I might just ditch my Note 3 for that, despite the fact that I'd be giving up on S-pen technology.