AKG C401 etc

[microx]

AKG make a series of little mouse like mikes for instruments. The C401 is described as an electret capsule in a sealed enclsure with almost no feedback problem. It's used as a contact mike. Another type, I think the C402, is meant to pick up from inside a sound hole and is not sealed.
Has anyone used these mics or seen whats in them?
Steve