It's an interesting bit of history. There is some documentation here: http://en.wikipedia.org/wiki/Nissan#Tie-ups_with_Austin_Motor_Company
You have to do a search on Nissan history, not on MG history as the deal was made with Austin.
In 1952 Nissan made a deal with Austin Motor company to manufacture cars under licence and to be able to manufacture components locally. From that agreement the Austin A50 was produced with a Nissan built 1489cc motor. This motor had a lot of interchangable parts with the Austin/MG power plant and once in a while one will show up in an MG sports car like yours.
The interesting point is that the Austin A series of engines as used in the Austin Somerset laid the foundation for the Nissan A Series power plants which are used all the way to current production.
Kelvin Dodd
Global Sourcing Engineer
Moss Motors, Ltd.
Helpful Links:
Code of ConductFAQsDisclaimer: Working on automobiles is inherently dangerous. Moss Motors, Ltd. is not liable for injury or damage due to incorrect installation or use of their products. All products are sold with the understanding that the safe and proper installation and use of the products is the customer’s responsibility. Follow factory workshop manual procedures and instructions, but use current shop safety standards and common sense. Some tasks will require professional advice or services which Moss Motors cannot provide.