I can help a little. But that is limited.
Since it only has one slide arm bar, that would be an early one because they later switched to having dual arms. I want to say in 1970 but I don't want to say for certain because I was never real great with dates and it's been a long time since I looked into it.
The other thing you mentioned is it not having a serial number.
That too would indicate an early gun.
Serial numbers were not required on firearms until the passage of the 1968 gun control act (GCA). So, the fact that it only has a single slide arm and no serial, tells me that it was made sometime between 1960 when they were first introduced and no later than October 22, 1968. Or at least not within about 60-90 days later to give manufacturers updated guidelines as to what the law consists of, but nevertheless, the GCA was signed into law 10/22/68.
This may not be all you were asking for, but it's probably as close of an answer that you'll ever get from anyone. Including Mossberg considering that it has no real identifying information.