Hi Wareling
I also own a 69 Fairmont XT and thus far I understand the 'plate codes' as follows:
The first letter in the body prefix (S) identifies the country.
The second letter (B) the specific assembly plant. Pity I do not know to which plant this refers to.
58-Fairmont
J-1969
M-March
I know that the last two codes in the second row (Z & 1.0Y) with regards to later GT models referred to trim and paint respectively.
I am not sure if this is the same for earlier models or to which trim and paint these refer to though.
'S' is for the high compression 3.6L six cylinder and 'B' is specifically for the 3 speed 'cruisomatic'. I am just mentioning this because these cars were apparently also issued with a low compression engine and a 'fordomatic' gearbox. I think the 'cruisomatic' is a C4 gearbox.
I found most of this info at the following link:
http://www.uniquecarsandparts.com.au/ford_falcon_identification.htm.
Were you able to locate any spare parts for your car?
I just recently started working on mine but I am already finding it hard to find parts locally. Attached please find a PDF with 2 pictures of the car.