The early iPower batteries were larger than a standard 9 Volt battery. Shortly after their initial release they made a smaller case for the batteries.
The new smaller 500 mAh units are marked the same as the older, larger 500 mAh units. The only difference I see in the case is that the older unit has a small recessed circular area only around the small positive terminal. The new smaller 500 mAh unit has a large rectangular recessed area that encloses almost the entire top of the battery. It's recessed about 0.030" deep. The new batteries are actually smaller than standard 9 Volts and should never be a problem as far as sticking.
We were told in early 2006 that it would be several weeks before the first new batteries were available. We only stock the smaller cells because we don't want them stuck in our units. If you order from some one else, I think I'd do it verbally to confirm what you want. If you have some of the larger batteries you might contact iPower and see if you can return them. Do a search on the RAMPS newsgroup for contact information.