TRP: The Robarts are unique in that they are the only ones that have a gear (or 1/4 gear?)which rotates the struts. This gear is mounted at the top of the struts, held on by set screws, and meshes against holes in the landing gear housings. Other manufactureres use different methods to cause the rotation. If yours has the gears, AND has 3/16" daimeter strut wires, then you have the 615s. If you got a complete set for $50, then you got a bargain. Despite my criticsms, they do work well IF you able to make consistent gentle landings. It is also a great help if the aircraft is kept as light as possible. As mentioned, my Top Flite Corsairs typically weigh less than 9 lbs. But there are a great many builders who end up with them weighing in at over 11 lbs.
And as for having them bind up as some have said, that is another builder issue: the mounting rails need to be parrallel. If not, the will case the housing to become warped, and thus cause binding. Regardless of which gear you use, set screws should be plentiful AND tight.
The parts that usually need replacement, "Links" as Robart refers to them, sell for about $6.50 each. There are 4 in a 2 gear set, so a bad landing can cost $26.
615s are the recommended retracts for the Wing Mfg. Hellcat.