Let me try to help.....
Ford assigned VIN numbers are not sequential....on the first day of production Ford did not build car number 1 then car number 2.....etc
Ford VINs were assigned by the computer...so they may have built number 2, number 7 and number 10...just a hypothetical.
Then you need to consider only one of every of every 71 Mustangs built was sent to AOSmith for conversion to Shelby.
These cars were loaded on rail cars and sent to Michigan. They did not keep track of serial numbers and once cars were unloaded they could have sat outside for weeks. The first car loaded would be the last car off, the first car parked in the holding might be buried, until production caught up.
Shelby did not assign serial numbers in strict numerical order. The first five cars built were Shelby numbers 41, 339, 101, 301 and 56.
So, once you take all those items into account its entirely possible....
It is correct to say generally speaking, lower Ford serial numbers equate to lower Shelby numbers, but not always.