The 2N6576, 2N6577, and 2N6578 are Darlington NPN power transistors from Motorola which are now obsolete. According to the datasheet they are identical parts except for the voltage ratings, please review the datasheet for complete electrical specifications.
Here are similar parts we carry that may work as replacements.
Discrete Semiconductor Products | Transistors - Bipolar (BJT) - Single | DigiKey
Please review the datasheets to ensure the alternative will work for your application.
Applicable Part Numbers
Digi-Key Part Numbers
MJ11012GOS-ND
MJ11016GOS-ND
2383-MJ11012-ND
2383-MJ11016-ND
2383-MJ11014-ND
MJ11016OS-ND
Manufacturer Part Numbers
MJ11012G
MJ11016G
MJ11012
MJ11016
MJ11014
MJ11016