Steve 100% mate
My father sold one maybe 10 years ago and got $300 USD from a gent in Michigan somewhere , it too had some scratches and a ding in the sheath and they have gone up since as he kept up with the values and often stated he regretted selling it when he did
his was marked with the diamond too , he was told that was a NCO ranked knife the plain where for the youngun's and as the rank rose so did the embelishments, i dunno if thats 100% correct though as there where different "brigades" of HJ (hitler judend) and some i read, had their group emblem on them
this is the exact same as the one my dad sold
it too still had the makers seal on it , it was never issued, it was found as part of the occupation forces with the Brit's , they found a bombed warehouse and his was found in there with quite a few others that where in even better condition , my father openly admits he traded his beer ration for the knife , he was outside in a bren gun carrier when all this stuff was found , he also had a unissued Knights Cross and a few other bits , not bad for a pint of beer ..
sold the lot for under a grand , now worth maybe 5-6 grand
and this is a page that will teach you a lot about them and copies