Bob, Suggestions, remove the back plate complete and turn it around and refit to engine, change the wires over, see if that helps to achieve setting the points and timing. You shouldnt really need to do the next suggestion but if all else fails take a small round file and elongate the slots a little so you can rotate the backplate a little further, put the back plate back to how it was if you like swapping the wires back over.
You mention the Condensers and that they seem o.k. in my experience Condensers are not to be trusted especially new condensers that are being manufactured these days, I stick to using old condensers that are maybe 55 - 60 years old, I test them by fitting to an engine one by one and see how they perform, some don't work at all, some appear to work o.k. but then fail when they get hot so its a case of giving them a good test, only do one at a time. Good ones go into a box ready for fitting when required. Don't be put off by the way they look, some of the most grotty condensers have found to work perfectly. they can always be cleaned up with a bit of fine wire wool then polished with autosol to a good shine looking like new.
Condensers can be checked before fitting to see if they are any good but I don't have the electrical equipment or knowledge of how to do the test, perhaps someone else can step forward and explain please.