YH6 is a free Bonus Value Option Package or Super Saver Package.
To quote the 73-87 RPO Guru KIILew:
The Sequoia Special was the spiritual successor to the Bonanza/Bonanza packages of the '80 and earlier time frame, which in turn descended from the even earlier Highlander discount option packages.
While the earlier packages were specific option groupings priced at a discount, however, the Sequoia amounted to the proverbial "buy several options, get one free" scheme, and were apparently offered during the winter/spring months. Specifically, the buyer would receive either C60 air conditioning OR MX1/MX0 3spd/4spd automatic transmission WITHOUT CHARGE if he selected the following options:
ZY4 deluxe two tone paint (not color specific, with contrasting color on the roof and lower bodyside)
V22 deluxe front appearance package (i.e. chrome grille/quad headlamps)
TT5 halogen high beam headlamps
U35 electric clock
A01 tinted glass
ZQ2 Operating convenience package (i.e. power windows/locks)
U58 AM/FM stereo radio (w/an upgrade allowed)
K34 electronic speed control
N33 tilt steering wheel
CD4 intermittent windshield wipers
The overdrive auto transmission discount was applicable only to 1982 and later 1/2 and 3/4 ton pickup models which normally offered that transmission on an RPO basis. The package content was likely fairly consistent from year-to-year.
Other than the reference to the "YH6 Sequoia Special" listed on your glove box label, these models are indistinguishable from similarly optioned (non-discounted) vehicles.
That'd being a 4x4 and higher than my 2wd they look more out of place on that truck than mine but still a solid square for someone to buy! 
