On the inside, they're the same. The bolt patterns on the front endbell are different, with the 540C having a similar pattern to normal brushed motors like in 1/10th scale, and the regular 540 has a square pattern.
SH Z-Car, Custom Crawler, 8s Savage, 12s XTM XLB 1/7 buggy, 4wd 4-link rear/IFS Pro4 truck, Custom Hyper 10 Short Course, Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz