ya, midsize is 110's but I don't know why they put that. That is definitely for a 50, and look at the title of it, it says :"Fast50s Billet Frame" , if it was for a 110 they'd say "Fast Minis Billet Frame"
that might be a 50 frame or a 110 but under fast 50s question board heres what i found
"DOES FAST50s MAKE A FRAME THAT IS BETTER THAN MY mild STEEL FRAME for my 50? Yes at Fast50s we have tried the aluminum frame route for our 50 line, but the aluminum unit bent far too easily under our test rider's constant abuse. Our quality control department would not allow us to mass produce these parts. We do now make a 4130 chromoly frame and swingarm combo that is a legitimate 5 times stronger than that stock (bend like butter) frame. We also have soo many other parts that will make your 50 a "Factory 50" Let us know what YOU need and it will happen!! (Just so everyone knows, we are in production of TRUE Billet Frames AND Swingarms for the 50's, 70's and 110's... Now your talking Super Duper Strong!!)"
there is a pic in a add in the back of either pitracer or mini moto mag the lastest issue of one of them i cant remember but ya it show one of the hayden brothers standing next to it and says 110 billet framed bike