Regarding deductible contributions, the instructions for Box 13 - other Deductions code R say:
Code R. Pensions and IRAs. Payments made on your behalf to an IRA, a qualified plan, a simplified employee pension (SEP), or a SIMPLE IRA plan. See the Schedule 1 (Form 1040) instructions for line 20 to figure your IRA deduction. Enter payments made to a qualified plan, SEP, or SIMPLE IRA plan on Schedule 1 (Form 1040), line 16. If the payments to a qualified plan were to a defined benefit plan, the partnership should give you a statement showing the amount of the benefit accrued for the current tax year.
Both employee elective deferrals and employer contributions to the partnership's 401(k) plan are payments made on your behalf and are to be deducted on the your individual tax return, so they need to be shown on the Schedule K-1 (Form 1065) as a pass-through deduction (in box 13 with code R). The only reason that a contribution made by the partner to the 401(k) plan would not be included in box 13 with code R is if the contribution was an after-tax (and therefore nondeductible) contribution made by the partner, and in such case it would make sense to show this contribution with code AH in box 20.
