All of the information that you've provided suggests this is indeed an offset distribution that is eligible for rollover with a rollover deadline of the due date of your tax return, including extensions.
I don't see how the lack of the code M that should be accompanying the code 1 would make it fail to be an offset distribution. As an example, the Instructions for Form 1099-R indicate that an offset distribution from a designated Roth account in a 401(k) that is an early distribution would not include code M in box 7 because there can only be two codes in box 7 and those code positions are already taken by codes 1 and B. I can only imagine that Fidelity simply deems correction to be unnecessary or has not yet implemented code M in their system (despite nothing in the Instructions for Form 1099-R indicating that using code M is optional).
As I mentioned earlier, it's up to the receiving IRA custodian to accept or reject the rollover. Out of curiosity, I looked at Fidelity's Deposit Slip available as a pdf online to see how they handled a deposit of an offset distribution and the form has not been changed to provide a way to indicate either a rollover of an offset distribution or a late rollover under self-certification, only a way to indicate a "60-day Rollover" (which in this context I'm guessing simply means an indirect rollover). In fact, so far I've been unable to find a form from any financial institution that allows one to indicate that a rollover is a rollover of an offset distribution (although I found one that refers to nonspecific "exceptions" to the 60-day deadline).
With regard to convincing the financial institution to accept the rollover beyond 60 days from the date of the distribution, it's entirely possible that such a rollover contribution is made during the same year as the offset distribution, but well after 60 days and well before any code M Form 1099-R is issued, so it seems like lack of a Form 1099-R with code M should not be an impediment to completing the rollover. Aside from convincing the receiving financial institution to accept the rollover, the only entity that has to be satisfied that the rollover is a legitimate rollover of an offset distribution, if they question it at all, is the IRS. The presence of code M would be helpful in convincing the IRS of that.
