I've been having a quick look at the voting records of both Frank Field and Jeremy Corbyn, and in the main, they are pretty similar, so was unable to see which one was the "true socialist"
The differences that I most noticed were that Frank Field voted for the Iraq war and against an investigation into the Iraq war, and that Jeremy Corbyn voted against the Iraq war and for an investigation into the Iraq war.
Jeremy Corbyn voted consistently that EU nationals living in this country should have a right to remain, and Frank Field was not entirely consistent on this issue.
Jeremy Corbyn voted against the benefit cap and Frank Field did not vote against it.
This last one for me, is the one I think, shows who holds the true socialist values.