Calling all Lepidopterists.

I found this wee crittur on top of a bucket of compost I mixed up yesterday, I think it must have fallen out of the birch tree.
When viewed from other angles it looks more as if the hairs belong to the caterpillar rather than spun as a cocoon, since the underside is devoid and, relatively "naked".
As the song goes "I've never seen anything like it in my life."