My guess (sheer speculation) as to not dating records is that they likely hit different markets in different time windows, and a dated product would identify it as an "old" release in a market crying out for "new" stuff. I imagine that most records were pressed in small batches, at least initially, to avoid the cost of over-production. The market place might not have been considered as too receptive of later pressings. Of course, "collectors" today would likely pay more for a record with an authenicated production or release date, but that probably wasn't a concern then.