I am trying to find out what the rules were when the age discrimination laws went into effect in 1988 with regard to late retirement benefits.

To make my question simple, what if NRD was 65 and an active participant was 70 on the date the new rules went into effect? Would they need to get actuarial equivalent increases from age 65, or only from age 70?

If I remember correctly, all years of service including years before the effective date, generally 1988 year, are counted for this purpose. It can be limited to years of participation. Requirement only applies to person who has an hour of service on or after effective date. Rules are different for DC plans.

I'm confused.
You only have to give actuarial increases if you don't give a suspension of benefits notice.
If you don't give the notice, the increase starts at NRA and continues until termination of employment (whether before or after age 70.5).
The law changed a few years ago (late '90s in connection with MRD changes but certainly not in 1988) to require actuarial increases at 70.5 even if you gave the suspension notice.