803B woofer will not be a direct fit in a factory Nineteen cabinet.
803B is designed for mounting to the rear of the baffle while the Nineteen cabinet is made for the woofer to mount to the front of the baffle.
803B also has a 14-9/16" bolt circle while the Nineteen is drilled for a 15" bolt circle which more or less centers the bolt heads in the slots of the 16 inch frame of the late model 416's.
If one were so inclined the 803B could be mounted to the rear of the 19 baffle with access gained by separating the top and bottom halves of the cabinet, and the T-nuts moved to the front side.
Or, The 803B could be front mounted to the baffle by cutting radial notches in the woofer opening to allow clearance for the reinforcing ribs on the back of it's mounting rim. Would kinda be a shame to cut up really good condition cabinets.
Elongating the bolt holes, or drilling new to accept the 14-9/16 bolt circle will be required in either situation.