Edit: It looks like if it's sequential, it would be a white plate, with California in cursive at the current (large) size, where the descender on the f almost touches the plate number, and it might have no subtitle or the sesquicentennial subtitle.
> DMV has the right to refuse any combination of letters and/or letters and numbers for any of the following reason(s): [...] it conficts with any regular license plate series issued.
My understanding is that all vanity plates are reviewed by at least one human, so it's possible they handle denying vanity plates that fit the sequential series that probably should have been denied in the online form.
[1] https://dmv.ca.gov/portal/wcm/connect/6d97bdef-c5b1-46ee-a55...