They are definitely interesting little peeps!

I can't say that I know exactly what color variation they are. However, I am not an expert and others may know more than I.
~My opinion~
The pattern looks reminiscent of a cuckoo polish (like you mentioned)... except that your polishes are abnormally dark for typical cuckoo polishes.
Silver laced chickens are usually lighter and more speckled on the wings so I do not think it is likely that they are pure silver laced. However, I think that silver laced is at least as likely as cuckoo at this point.
As for mottled... I have never heard of a true b & w mottled polish (that is basically what a Houdan is... except Houdans usually have 5 toes and are somewhat hard to come by). The closest thing to a mottled polish that I know of is a Tolbunt polish... but Tolbunt polishes have at least a little brown on them.
The last possibility (in my opinion) is that they are a mixed color variation of polish. However, they are still young and may exhibit a more obvious pattern after the next juvenile molt... which shouldn't take to long since peeps grow like weeds (very cute little weeds, that is

).
Sorry that I cannot be of more help!

However, If it's not a bother, I'd love to see pics in a couple weeks when their feathers come in s'more.
