California income tax (paper Form 540) requires attaching a copy of the taxpayer's federal return. If, when filing a given year's California Form 540 for the first time (not amended), the federal return has already been amended, then what exactly should be attached?
The original federal return includes Form 1040 and associated schedules, with some incorrect information. The amended federal return includes Form 1040-X (summarizing changes) and only the schedules that needed correction. Thus, the original federal return still seems important to provide to California, because not all federal information is captured in the amended return.
It seems that both the amended federal return and the original federal return should be attached to Form 540. But if all these federal forms and schedules are attached consecutively in the same mailing, it seems likely to lead to confusion about which is the newer correct info and which is the older incorrect info. So is the best practice to add a label across the top of each federal page saying either "Amended federal return" or "Original federal return"?
A possible alternative would be to create a single, notional, complete and correct federal return (Form 1040 and all relevant schedules) as if it had been correctly filed in the first place, and attach that to Form 540. However, this is not the format in which the info was provided to the IRS (since it is not the way the IRS directs amended returns to be filed), so it might appear discrepant or fictitious, and it would require additional work.