theres also differences in wheel slip and slide protection.
A 508 has 2 compressors, one, in one of the DMSOs is the same as the 507 compressor. The other is under the TOS and is the same as a southern slam door unit compressor.
The one in the DMSO makes a whirring buzzing noise, the one in the TOS makes a thumping noise.
Theres also slight differences in the cabs, the 507s have a larger, bulb lit speedo and brake gauge whereas the 508s have a smaller plastic surrounded speedo and brake gauge (think 455, sprinter etc) Which I believe is luminous.