Not really DBotkin. The old Model 28 doesn't have perfectly flat sides... especially around the barrel... at least mine doesn't.
The frame wrench should really be inlet (at least partially) into hardwood blocks that are fairly thick. The 'wrench' should be flat jawed and completely cover the wood blocks. I have an old plumbers wrench that does not have any serrations cut into the jaws and I believe that it might be what you need to look for. The serrations in a standard plumers wrench might crack the wood.
Here's an image I pulled off the internet: