The 3600Q-1BNC (predecessor to the IS503X switches) is listed on the EOL (end of life) page.The link below will let you download the latest SW image for this product.The SW image contains the FW and is automatically updated with the SW upgrade.To enable the SM you would need a FIT license if you have one the command to enable it is "ib smnode <your switch name> enable" example below.Be sure to do a "write memory" after changing any configuration so as to allow it to be persistent on reboot.
IS5030 [standalone: master] (config) # ib smnode IS5030 enable
Link to last GA version 1.1.2700 SW-
Isn't the BNC model managed by default and thus includes the Subnet Manager? at least that's the way I read the manual here:
Page 22 says the subnet manager is included for internally managed systems, and page 6 shows the BNC code means Managed IB RoHS.