Actually I was looking for 6 digits such as all my other Wix filters. For example I have a 51515 with:
061406C1, which I believe is June 14 2006.
So, looks like they changed it long ago. It's a 2000 motorhome. They must have changed the oil again after the filter change because when I got it the oil blotter was like brand new oil. Ran it 33 hours that way. Just put a new Wix on it and filled with Redline 10w30.
51394 is the filter specified for my Nissan application. I have tried this filter on two separate occassions and have had valvetrain ticking both times. Swapping out the filter with an OEM 52J00 Nissan quiets the motor down.
While the filter may be very well constructed, it definitely does not make my Nissan happy (assuming it likes to run quiet).