That isn't a comma, that's a mintmark of "D" indicating it was struck at Denver mint. I believe there are a few solutions for restoring dates on these, but having had zero experience of doing so I'll leave it to the other members to input here what is best. Honestly though the cost of buying any special solution would likely vastly outweigh the combined value of them due to their overall poor condition. I believe soaking them in vinegar does the job too, but this isn't an advised method and it's probably best to check with other members if the value would be so far depleted due to condition that this might be fine just for curiosity purposes.
Bringing out the missing date with an acid solution like Nic-A-Date will answer the question of the date of the coin, but it damages the coin in the process. It works by removing copper and leaving the much harder nickel. (I think that's right.) If you have to know the date, that's probably the best you can do. These two coins are really worn, so you can't do much harm.
"Shine, shine, a Roosevelt dime All the way to Baltimore and running out of time" - Tom Waits 'Clap Hands'