You can spot the bridge on the east side of the McAllister Springs Conservation boat ramp. There is a short,vague walking trail on the east side of the conservation parking lot along a farmer's fence line and then along the Blackwater River. The road that once went to the bridge is long overgrown with woods on the south side and a farmers field on the north. No visible road.
From looking at old MoDOT maps, it would seem that the state did not take over maintenance of this section of the road that includes the bridge and is now known as Route K until 1963 or 1964. Initially, this part of the road was designated as Route EE, while the section of road to the south of the Pettis County Line (which had already been under state maintenance for a number of years) was known as Route M. It looks like the whole road was designated as Route K in either late 1975 or 1976.
It would seem the state decided to build a new bridge (which would have been normal procedure in most cases) and took the opportunity to straighten out the alignment at the same time. I suppose there is a slight possibility the old bridge was initially left in place to serve local residents/landowners. The more likely scenario is the state and/or county simply didn't want to spend the money the tear it down.