This Manfrotto R354.08 Locking Lever Assembly is included with the compatible tripod heads, and is sold here as spare or replacement parts.
The release on my head was not automatically releasing as it did when it was new. I contacted Manfrotto, and decided to take a shot repairing it myself rather than sending it in (which would have cost more in shipping than the cost of these parts).. After replacing the parts, I still had the same problem. What I discovered is the nuts had spun over the course of time, and as a result, the spring was inactive. After discovering this, I turned the assembly to tension the spring and it worked. I now have marks on the mount and the nuts so I can tell by looking if they have twisted out of position.