Hi Wurger,all Jv44 lower surfaces are Red with the white striping as the Eagle files 1 shows as well as the Red numbers. I believe Jerry Crandall's research and The Japo D-9 Vol 1 leans toward this as well. Earlier info from Experten decals indicated Black lower surface and Black 1 but this detail has long since swung in favour of Red. A couple of colour photo's of Red 1 have been printed in Luftwaffe im Focus No.7 and appeared on the net some time ago.
There are supposed details of other aircraft numbers and variations on the undersides but these are yet to be proven via photo's.
Are you aware that the Red 13's number is different on each side of the fuselage for this aircraft. Photo's are in the Vol 1 from Japo, you got to get this book...save your pennies!! Eagle Files 2 also has the extra shots.
Is the Black detail you mention new info or has it been around for a while, I'm curious, what is the source of this info?