Away from books so I can't look at it. But, does it have a damage code as well?
If so, it sounds like brass knuckles, but for your feet..... and if so... its probably to answer the great question that was asked exactly once:
"Why can't I wear brass knuckles on my feet for my kicking martial art?"
Followed by the same person asking: "Why can't I get a weapon focus for my feet??" 
Sorry, I don't follow yer there, mate?
Dmg code is STR+2 and the description is literally "knucks for your feet":
"Also known as “drek-kicker” boots, these specially designed combat boots not only protect one’s feet and look stylish with tactical gear, but with their reinforced soles and
metal toes they also act as knucks for kicking. That’s right, with these boots, kick attacks are more damaging than ever. Perfect for kicking in doors or a perp’s guts!"
The Plasteel Toe Boots for HT is STR+1 and use physical limit, so I'm confused as to why these wouldn't. They also have an insane -4 AP, which may be why balance-wise, but still doesn't make sense.