I think you may be right - I took a bad picture of mine last night and I think it says "...at altitudes at or below 4000 feet." I'll have to look again tonight.
In 1977 I don't know if there was an extra high-altitude compensation (HAC) valve, maybe Sascha knows...
Rising Sun Bio
2F "Brahma" + Lockright, tach, Warn 8274, FJ60 Power Steering,
no more Sanden OBA
1FZ-FE factory lockers +
Safari intercooled turbo, CDL switch, cup holder, Metric TLC leather, heated seats, JDM switches, PUMA OBA.
Warn M12000 waiting for a bumper.
Limited TRD 2UZ-FE SOLD