LB--your cover is definitely a rotary press issue. Most probably, (with that very late use of 1939), a 496; but an outside chance of it being a 458. You would have to lift the stamp to check the watermark to be sure, but I wouldn't really recommend it unless you just have to know. Since the cancel is so light, lifting it and then hinging it back in place could cause some concerns as to whether the stamp originated on the cover, should you wish to part with the cover at some point in the future.
