Thanks much for your help.
The manual refers to something called a "striking lever" that is held by a lock pin on the "striking rod" in the rear extension assembly...
perhaps this is it

Sounds like the selector finger is bent. It is not unusual for someone who has recently resealed a nissan box and tried to get out the cotter pin that retains the finger to replace the O ring from the shifter sleeve. They usually try to heat up the finger to get the pin out and weaken the finger so that when you do a hard shift, the finger bends and you lose either all the top gears or all the botttom gears. When you are really unlucky you lose all but one gear. Then you are stuck.

I would not say easy. Depends on how careful, and patient you are, and how well you pay attention to details. You will need the factory service manual (1977 or 1978 B210). Examine it at the regional library and if it doesn't scare you, dig in.

I am only guessing at the cause. Maybe someone else have different idea.

Thanks.

Is this an easy repair or a tranny shop only deal?

Bent shifter fork (internal part).

Recently installed a 63A transmission 5spd in a 76 B210 and got about 150 miles on it before it lost R, 2nd ,& 4th all at same time.
main synchro? anyone have an idea about what to expect for a fair price on repair?

Thanks.