Yet another VIN deciphering question

[El Campo]

On my project "Mistake/Attempt #2" I am trying to figure out what the heck frame I have under it. I had to modify a lot of the mounts on it and build my own bumper brackets to get the 72 front bumper on. The VIN on the frame (on top of the driver's frame rail by power steering pump" is the following:

6S 56999

Any ideas what this makes it? I have read on lots of posts on here with no
Luck.
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[RenoKeene]

On a Chevy the first digit is the year...1966
Thats if it is a Chevy frame, GMC's are different.
My best guess..but I'm no expert.
Do any of the last digits match your VIN?

[El Campo]

Have no idea! Found the frame out in a field and it was a short bed frame and had trailing arms and a manual steering gear. That's it. I'll try to get pics of the front frame rails as they look strange to me.
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[special-K]

It's sounding like a '66 to me.

[Shyguy]

The pre-67 front frame horns are different and to get a '67 on bumper mounted to them have to cut and reverse from side to side if I remember right, I could be wrong since it has been quite a few years since we put a '69 on a '63 frame.

Also I think we had to move the cab mounts to make it work.

Does the frame have the dimple for the steering box, '68 on did?

[El Campo]

Hmmmm not sure what you mean by a dimple. I do remember that it does not have the same mounting fashion as my 72 frame did. The 72 has the little "hills" that the bolts go through that hold the steering box on and this one is just flush. And I had to drill different holes to get the 72 steering box mounted up. Ill try to take pictures next time I am near it.