I think I am right on this one, but the IRS information is really difficult to read
An employER can make a previous year contribution to an employEE's SEP up to their tax filing deadline (plus extensions). An employEE CANNOT make a previous year contribution to a SEP. Isn't that correct?

I didn't think employees can contribute anything to a SEP.

If I remember correctly, the individual could have an IRA account that contains traditional IRA deposits as well as SEP contributions. You need to segregate deposits by SEP vs. traditional IRA. Employees can make contributions for a prior year up to tax filing deadline.

an employee cannot make a SEP contribution at all. they can make a traditional IRA contribution into the same "SEP IRA" however. Those contributions would be subject to the same restrictions as any other traditional IRA contribution.