Usually stamped into the end plate where you suggested it should be. Every original motor I have ever seen has the number stamped there - RS or open headlight car. If you have no visible stamp, or even a trace of one showing, it might possibly be a replacement motor. Stamps are heavy enough to be seen even under some pretty thick top coats.
I assume your motor has the headlight washing system ? Date codes on originals took two forms, a silver label with the part number and date code, and/or an inked date code and part number (most times yellow) on the side of the case. I have and have seen examples of both.
Include a close up pic of your end frame for our fanatical eyes - and include a shot of the edges of the motor case. Late replacements and imports use a different case assembly fastener that's easy to spot.
Regards,
Steve