I bought some that look about the same as yours from harbor freight, they work pretty good but I use them in the basement (cement floor) probably wouldn't work good on anything that's not flat

Get the real thing:

http://www.marcospecialties.com/pinball-parts/PBS

I have a set I purchased years ago when they just came out...I would NEVER be without a set..it is money well spent!

I also have had a set since they were a new product. Purchased from Marco also. I have owned and moved a pile of games. Excellent product.
Harry Irvin

On most surfaces you can put the skate on the front leg and slide the machine outward to get to the back legs. Worst case is using rubber cups on tile, then you might have to crawl under to put the rear legs up on the skate. Generally if I have a game at a show where there is tile, I only put the rubber cups under the front legs. It keeps players from being able to dance with the machine, but leaves the rear legs free to slide.

I love the skates in combination with a Pinball Lifter. The skates get the machine from point A to point B no matter how many ramps or curbs are involved. The lifter gets the machine in the air and left you take the legs off/on to put the machine into or out of a truck.

Just a tip. When you lift the bar that has the slot that secures to the leg of the pinball, I usually have a shop towel in the mix. Keeps the bar from scratching up the legs.