According to Clive Carefoot marbled chickdown is characteristic of the genes that contribute to spangling.
ER with pattern gene, Db and the melanotic gene, which gives the type of patterned tails seen in silver spangled Hamburgs. The alternative is a black tailed spangled bird as in the gold Hamburgs.
This year I have hatched several of these chicks with marbled chickdown.
Some of the chicks are now maturing with proper spangles and spangled tails.