Autozone distrubutor vacuum advance locked?

I installed an Autozone reman electronic ignition distributor spec'd for '72 340 Demon when I swapped in my Magnum mill. It's advancing a bit much much at 4K RPM and I'm getting some ping, so I break out the 5/32 and go to adjust the vacuum advance canister. It won't turn! The wrench engages, but it's locked...tight.

Has anyone else had this problem? I bought the distributor a few years ago so I can't really return it unless I get lucky and its lifetime warranty. But, I might swap vacuum canisters unless someone has some advice.

Thanks
John