I've looked at the plaques on these bridges and occasionally wondered what the numbers mean so I did a little research and learned how they are numbered.
The system was created in 1953 by John Diehl of Cincinnati. The first number designates the state in alphabetical order in which the bridge is located. Alabama being #1 and Wyoming #50.
The second number is similar but it represents the county in which the bridge is located. Ohio counties start with Adams county at #1 and Wyandot county at #88.
The third number is the number given to the bridge itself. Although there was no real system to that. It seems though, looking at the numbers of the bridges that I documented, that they were numbered somewhat geographically.
Here's a good link if you like learning about such things.
Be patient the page loads a little slow. The system, developed for Ohio, was adopted by the National Society for the Preservation of Covered Bridges (NSPCB) when it published its first World Guide to Covered Bridges (WGCB). I found it very interesting.
The Bell Bridge for example................
State #33....Ohio, county #84....Washington, bridge #12.
"Aha!!", those of you who know that Ohio is not 33rd in alphabetical order will say, "It's 35th." FAIL! The system is flawed! I had to think about that a little bit. Then I considered the year the numbering system was developed....1953. In 1953 there were only 48 states and Ohio indeed came in at #33. It wasn't until 6 years later that Alaska and Hawaii became states and that put Ohio in at #35.
Only one of the bridges, The Rinard, which I visit a little later, shows Ohio at #35.
There ya go.