All 2003 MMs whether 300A or 300B had the Grand Marquis "M75" in their VIN. It was the "V" engine code that distinguished the 4.6L DOHC engine of the Mercury Marauder. That includes both the 300A and 300B for the 2003 model. VIN doesn't help in the A-B discussion at all.
The Marauder unique VIN "M79" was introduced with the 2004 Mercury Marauder also with the "V" engine code. BTW -- all 2004 MMs are 300As (but with the features of the 300B from 2003).
You are correct if the owner is not one of the original owners who bought the car new. Some of these cars have changed hands several times in the past 10+ years and may have replaced the spare. If you know what you are looking for, you can determine whether it is a 300A or 300B without looking in the trunk anyway. It is very easy .... don't know why some folks seem to have difficulty figuring this out.