, they only made them for 12" and 9".. The 12"s are obviously beyond rare.
for the tyco pro era they only had 9",12" and 6" radius curves, only 9" and 12" got the aprons, as optional.
there was only a few sets that came with a few pieces of aprons sections you mostly had to buy them 'À la cartet'
the aprons were designed as 1/8 pieces so you needed 2 for a 1/4 curve.
I don't think tyco ever made a 15" curve, I own a 15" tyco type connector curve but it was made by somebody else not tyco.
for the later tyco only the U-turn sets used aprons because they "U-turned" on curve sections only. but those only came as 9" and 6" aprons.
the 6" was used for the inside lane, and 9" for the outside lane. they had keyed left and right pieces and came only with special radius track.
now you can get the 12" tyco mattel track used on the dirt bike tracks that has a built in outside lane apron.
what i found was i bought the special 15" track from Evil-bay, but its still not enough as far as room for throwing the butt of the car out. So i went to the 12" with the apron, and it was night and day, the apron makes all the difference in the world. I went back to the 9" and installed aprons, and again night and day difference. After that i scrapped the new track and went to the tyco pro track, Everything has been night and day, I put the 15" track away and now either use the 9/12" with aprons or go to the monza style 20 degree banked track or the full 40 degree high banked turns with aprons.
I really scratch my head because the old stuff is amazing to bad its made of chalk and glass(breaks very easy, no lateral support)
whats cool about the aprons is how you can use them to balance the speed of cars in different lanes. if you find the inside lane is faster, just add a few aprons in certain areas for the outside lane, and you will balance the game play. you can even put them in places to cause a delay. for example you have a down hill area where the outside lane seems to be faster, you trick the player, install the outside lane aprons but right at the end of the aprons you put a guard rail. you have to seriously slow down at the climax of the turn to avoid hitting the rail, so if you get a lead foot coming out of that corner he either deslots or slows down as the inside lane is drifting past.
REALLY cool stuff you can do.