Invoice Month/Year = Model Month/Year ( That is correct)
Delivery Month/Year on PBO = Tentative Delivery Month/Year = Registration Month/Year (unless registration is deferred to wait for next year) [ i didn't get this equation of yours ]
listen if December tentative delivery month is written on PBO then it means that IMC commit with you that they will deliver your car in December month it means that definitely your car invoice will be generated in December month because unit is shipped/transport after the car invoice is generated in the name of the customer.
and also Tentative Delivery Month/Year may not be equal to Registration Month/Year as if your car invoice is assigned on 30 January and you will receive your car in let say 2nd of February (or your bank will receive invoice document on 2nd of February) then obviously you or your bank will register your car in your or bank name in February month