MG T-ABC

I have a NOS Moss crankshaft here (451-490) that just came out of the box with a bit of a surprise. Has anyone seen this before? Possibly a balancing thing? I was planning to put this into an engine with a Moss rear seal kit, so obviously this won't work unless anyone has any bright ideas? If not then I guess it will have to go into an engine without a seal kit.

I held off thinking someone might have a more informed answer.
Balancing would normally be done by grinding off a bit at the counterweight throws opposite to the crank pins, i.e. opposite to where the con rods are attached.
Moss now offers 451-485 billet crank, which they say replaces 451-490 forged crank.
I have no guess about a reason for your notches.
It might be possible to fill those notches with weld. Heat the crank first, then weld a little bit at a time, let it cool, then a little more. Then machine it off smooth.