Does anyone know if a mandatory employee contribution to a money purchase pension plan (as a condition of employment) is subject to FICA or income tax? In other words, is the contribution treated as an after-tax employee contribution or an employer contribution (which is not subject to income or FICA taxes)?

After-tax employee contribution.

If the employer is a govt entity, mandatory contributions can be excluded from income tax as an employer pick up under IRC 414(h)(2).

We have several clients with postal trucking contracts who have money purchase with required contributions and these are NOT subject to FICA.

USPS contracts are under the Service Contract Act (DBRA) and have their own prevailing wage rules that are not applicable otherwise.

I agree they are after-tax contributions -- no FICA or income tax exemption applies. See e.g. Public Employees Retirement Bd. v. Shalala, 153 F.3d 1160 (10th Cir. 1998) and PLR 20030500.