Justin,

You can probably narrow the search down to a few years with just the information on hand.  First check out Keene Studio and find out when they were in business.  A quick Google search (the interwebs knows all) says that George Keene got out of the photography business in 1921.  So already you have one firm date on the gentleman.  You just need to narrow it down from there.

Next, find a badge collector and look through their collection.  Chances are the badge was a stock order item and styles changed over time, so that will give you another range.

Next, check out the uniform itself.  I’m sure there are sources for vintage clothing on the internet that can steer you in the right direction.  If you’re still in Mankato, check with MSU’s theater department.  They’ve got a pretty good one and any decent costume designer will be able to help narrow down the era.

Next, do the same with the hair style.  Not as accurate as the other methods, but every little clue helps.

Next, check out the U.S. censuses on Rootsweb.  You may have to get a subscription to look up the info, but this will reap big rewards on ol’ Lenny.  Check the 1920, 1910, and 1900 censuses.  Chances are Len lived in Mankato for quite some time as he appears to be in his 50s and was most likely a long-standing member of the force.

Hit the Blue Earthy County Historical society and look in old Mankato phone books for Len.  Search the newspaper archives at same and find an obituary for Len.  If Len moved sometime in the ‘20s or ‘30s, that’ll show in the census records.  As a sergeant, he may also show up in newspaper stories from the time.

I see from your web site you’ve already traced down the studio.  You’re almost half way there!

-Todd