I'm not sure but my guess is that Martin bought Fairmount body tools then moved the manufacturing to China. This allows them to compete against "off-shore" brands.
Gibson guitars did this with Epiphone. Edelbrock sells the E-Street line which is also made in China.
I just found out that Harbor Freight has the window liners for $1 each. It seemed expensive until I checked out Eastwood. I have the smaller HF cab that has a 22.5 x 10.5 window Model 42202-7VGA.
In the manual, available on-line, the page is 6 and the part number is 22.
To order you have to...