First - it depends on -number of the Mustang for production change from fabric to metal elevators. The P-51D-20-NA at 44-63560 and P-51D-25-NA at 44-72627 had them installed on the line. The last D-20-NA w/o metal elevator was 44-72626, which was the last D-20 before the first D-25 (44-72627). I would be careful showing fabric for the D-20's, however because they were almost certainly modified at an Air Depot before deployment to combat operations.
That said, all fabric covered elevators were retro fitted in the field.
That said, the period represented and the dates could show fabric through November/December for all P-51D-5, D-10 and D-20's (lower tail number than 44-63560 or 44-72627)
If you see the early P-51D-5 w/o DFF you can be certain it was a fabric version.. or any period picture from May 1944 through December 1944 you can safely portray fabric.