Nokia 525 was released as an upgrade to Nokia 520. The only difference in specificaitons of these two phones is that Nokia 525 has 1GB RAM while Nokia 520 has 512MB RAM. External difference is that Nokia 520 has a matte finished back panel(with black NOKIA lettering) while the 525 has glossy back(with subtle NOKIA lettering).
Other than that, both phones have the same specs, i.e., same processor, resolution, display type etc. Even the external dimensions, weight, button layouts etc are also exactly same.
So the silicon covers, back panels, slipon cases, hard cover cases, screen protectors, bumper cases or whatever are interchangeable between Nokia 520 and 525.