My first JJ had a new 18.5" standard kettle grate in it, and it did not fit (fell into the bowl).
My second JJ had the correct grate and it measured 0.25" larger across the diameter compared to the standard 18.5 kettle grate.
Good news is I think the old JJ grates can be ordered from Weber.
I have read where the new JJ uses the standard 18.5 diameter kettle grate and can even support the Cracort CI grate.
Hope this helps.