The lock looks a good one with heavy brass case, detector and curtain so it's worth a look on the top of the lock salvage (hidden by boltwork)for a possible serial number- you might be lucky. For the sake of 2 mins to remove the split cotter pin and slide the boltwork out you never know it's more likely to be there than stamped hidden on the back of the case. Without any numbers to work on I can't see much happening to pinpoint a date anymore accurately than you have tbh.