Here's your serial number break down, which you probably already know...
H53S01703
H = 3100
53 = year
S = St Louis
01703_ = unit number
The unit numbers started at 001001, so you are defiantly missing a number. If the DMV looked up each of the 10 possible ending numbers they could see if the other 9 were still in use and rule them out... Without doing that there's no way to ever really know what the missing number is/was.
Maybe Missouri would tell you who had the truck before their last known guy so you could work backwards in hopes of finding the whole number from a past owner?
Grabbing straws here...good luck!
Mike B
