Form 1040-NR does have some instructions about allocating pensions between earned income and investment income.  But it seems to be based on when the employment occurred, and 1986 was a relatively long time ago.  Where did you find something more specific?

If you worked in the United States
after December 31, 1986, the part of
each pension distribution that is
attributable to the services you
performed after 1986 is income that is
effectively connected with a U.S.
trade or business.
Example. You worked in the
United States from January 1, 1980,
through December 31, 1989 (10
years). You now receive monthly
pension payments from your former
U.S. employer's pension plan. 70% of
each payment is attributable to
services you performed during 1980
through 1986 (7 years) and 30% of
each payment is attributable to
services you performed during 1987
through 1989 (3 years). Include 30%
of each pension payment in the total
amount that you report on line 5a.
Include 70% of each payment in the
total amount that you report in the
appropriate column on Schedule NEC
(Form 1040-NR), line 7.