I just went through this when I was looking for a starter for my 550. There are three different starters used on a 540. Early '58 through serial # AM1355 used a 6v. with a stamped steel mounting plate. Late '58 & '59 (AM1356-AP1000) used a 6v with a cast mounting plate. '61-'62 used a 12v starter. Phil Heisey had a 540 starter a while back but I don't know which one it was. Give him a call at (215) 256-8651 or e-mail: philnoreenh@verizon.net.  If Phil can't help you, try David Sales  in L.A. (google it)  He has a barn full of obsolete starter and generator parts. He had (NOS) what I needed to repair my 550 starter.   Good luck!!,