Factory auxiliary fuel tanks ? Surely not................

[RSRXION]

Ok guys, I can't stand it anymore. When I bought my hauler I noticed that it had a fuel tank switch mounted just under the edge of the drivers side seat. I just asumed that it was an add on that somebody had installed and since my hauler only had one massive tank under the bed, I removed the switch and threw it away.

Then last week I sold my GMC parts truck to a friend of mine who had a twin GMC with the exception that his truck is a 3/4 ton. As I was looking his GMC over I noticed that it had a pair of aluminum fuel doors on the bedsides and that same funny looking little switch in the same location as my car hauler . Hmmmmmmmmmmm............................

Then today I go pick up my 71 1/2 ton parts truck and to my surprise I notice a pair of little aluminum doors on the bedsides , a couple of tanks visible just under the edges of the bed and that same little switch in the same location !

What gives ? Surely this isn't a factory install................... IS IT ?

Thanks, Dean