[Model51]

flathead34
If you want the petcock to work, it's probably best to remove it, disassemble and clean it. Heat may not help as the corrosion preventing it from turning is an oxide of brass on both parts and doesn't quite react the same way as iron oxide does to heat.

Also, be careful when you try to remove it from the block. The section of the valve which goes into the block is hollow and your are going to be fighting brass-iron corrosion. If it shears off, then you have a much bigger project on your hands.