Brendan,

There is a resource you can use to de-code those alphanumeric codes on Rand McNally, H.M. Gousha and similar map company maps. 

Go to the Western Association of Map Libraries' homepage and you'll see a Map Librarian's Toolbox there: http://www.waml.org/toolbox/index.php/Main_Page (keep this for use in the future, its a great resource)

From there do this:

Cataloging & processing -> Map Dates -> click on the header "Map Dates" and it takes you to another page, then choose "Rand McNally", you'll see about 2/3rds down the page a link to "Dating Rand McNally and H.M. Gousha Maps", click on it.

This takes you to the Road Map Association's page with a table that you use to figure out dates based on the letter (first) in the alphanumeric code on your map. 

Now, after all of that when I look at the info you have given these may be "too new" for the chart (though if you scroll down there's a link to apparently a further explanation, you might want to check that out). You are on the right track by using the population data given on each map. If you are unable to de-code the alphanumeric code at least you have enough population information to narrow things to questionable dates either at the decade level or the year for each. 

Hopefully, if you are searching for copy in OCLC, a fellow cataloger has included those specific codes in a 500 note so that you can tell if you found matching copy or not. Its possible that another cataloger has done the legwork already and came up with a viable solution to both the publication and date of situation dates.

Paige

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