Depends what the jack on the 6505 switch is - if it's just a TRS plug, then it should work (pretty sure the 6505+ uses a 7-pin like the JSX and XXX, not sure about the regular 6505)
I actually still have the footswitch from my 5150 combo that I had ages ago, I can check on my 6505 to make sure it still works. If it does, I can send it to you if you cover shipping
I've had both the sig 5150 and the new 6505, as far as I can tell they are the same...the only difference is in the connector itself ( that's just a better stereo jack connector)