4x4 question

[pimperout]

i found a 79 chev truck at my cousings house which he said he would sell me for a pretty cheap price, so i went and had a look at it first thing i looked at was to see if it was a 4x4 bye the locking lugs. it didnt have locking lugs then i figured it sat wrong to be a 2wd so i looked underneath and sure enough it was a 4x4 of some sort. soo i asked why it had no locking lugs with a front diff, i thought mabe it had auto locking hubs aftermarket or somthing, but no he said its some sort of factory all wheel drive. just wondering if this is anactual factory option, how it works, and if its rare. thanks for taking the time to read all this i know its a long explainaion lol.