Not a historian, but I have cataloged a similar collection of photos for a historic institution. The practice of corpse photography was alive and well in North Carolina in the 1950s. I would say that at least 75% of the bodies were less than one year of age. Some of them may not have been photographed before they passed away. There are volunteer groups today who photograph stillborn babies for parents, apparently it helps the grieving process.

I did see images of several adults, laid out in coffins, taken between 1945 and 53. This photographer documented car accidents for insurance, but the corpses were in formal dress, the images didn't appear to be related to an investigation. There were no business records to indicate who commissioned the photos.

I've worked through several personal photo albums, none contained corpse photos, from any time period. However, the albums that get donated and accepted by a historical institution are all formally structured, and reflect on important families or events, so I can't speculate on how many people took snapshots of funerals.