I am having the same problem. I believe our customer sent the project file (for a v4 model) to us as a *.dat file, but I don't know how to extract the project file infomation from this .dat file.
but unless you have V4, you want be able to get the PRJ from the DAT file. V5 will only extract .model and .session from DAT file. (correct me if I am wrong)
You can try the utility in V5 called ExtractModelFromSequential
if it does not work, ask your customer to send you PRJ as ZIP...
indocti discant et ament meminisse periti Eric N.