Puttygen source code

Private-key formats aren't as standardized as public-key formats because, ideally, they are never sent or received— so interoperability isn't as big of a concern.

However, documentation on that format can be found in a couple of places. It's very similar to the RSAPrivateKey structure described in PKCS#1, which would normally be carried inside a PKCS#8 container. However, the normal way of storing a DSA key in a PKCS#8 container is just to store the private value x, not the whole structure that you have there. I'm guessing that it's a proposed-but-never-technically-standardized DSAPrivateKey format which OpenSSL implemented before the standards settled down. The OpenSSL dsa man page says:

The DER option with a private key uses an ASN1 DER encoded form of an ASN.1 SEQUENCE consisting of the values of version (currently zero), p, q, g, the public and private key components respectively as ASN.1 INTEGERs.