Has anyone ever run into an arrangement that according to the document is a DB and a DC plan under §403(b)? The client indicates that these programs are highly unusual. The Form 5500 is completed as if it is a 403(b) arrangement but the "codes" include both DB and DC features.

Perhaps one document written to encompass two different plans?
(BTW, not a good idea.)

"Defined benefit" and "defined contribution" are technical terms that describe characteristics of I.R.C. Sec. 401(a) qualified plans. The language doesn't apply to 403(b) law. A 403(b) plan uses Code 2L and/or 2M on Form 5500. You can find more information on page 9 of the instructions to the 2003 Form 5500.

Yes. Long, long ago in an annuity contract far, far away I had a client who maintained a DB plan where employees made pre tax contributions under 403(b). Since the plan was established under 403(b) no determination letter was issued, plan paid DB benefits and filed 5500. This plan was established pre ERISA and client assumed that it was grandfathered under the 403(b) regs which under the now repealed exclusion allowance rule provided for reduction of 403(b) contributions where employee was covered under a DB plan. There is nothing in the in the 403(b) regs that prohibits a DB plan.