ANA 513 Engine Gray is the color you're looking for. FS 16081 is an almost identical match. GSI Creos Mr. Color Engine Gray (16081) 339/ Mr. Hobby Engine Gray (16081) H339.
HTH,
DB Andrus
Apparently this was a backup a/c for Hartmann (see number 2 inset in the larger number 1). Does anyone have the work number available? Thanks, D.B. Andrus