Tax consequences of a grandfathered 457 plan

[smm]

My question concerns the taxability of grandfathered 457 plans (i.e., pre-1986 plans for tax-exempt entities). How are amounts under a grandfathered 457 plan taxed? Is the taxability governed by the substantial risk of forfeiture requirement or are the amounts taxable when distributed/received (i.e, assume an installment payment schedule).

Thanks.