Jim King wrote:

The on-line list of Martin's kits shows 39.8, a MP rebuilt Howe truss box
with diagonal panel roof and inverted Murphy end but doesn't state car
length. On ebay, kit 39.9 (which is the same as 39.8 except for decals) is
shown and that label says it's a 50-footer.

Jim, you're misreading the label. It doesn't say 50', it says '50. That is, it's the 1950's blue & gray Eagle Merchandise version of the car, which, as others have pointed out, is 40'.

Tom Madden