After a quick look at the files in the old group it does indeed look like a lot of them are missing. I'm not quite sure why this is, perhaps if users leave the group the files they share disappear? Regardless, unfortunately I'm not sure you can get them back, as far as I know we haven't kept any local copies of these files. What is it your after? If it's something someone on the gallery posted they may be able to help you out.