According to the Lyman recall notice the guns made prior to serial number A595960 are safe to fire. That serial number translates to a manufacture date of 1 March, 2017. What changed on that date.
Most likely, the dates quoted in the recall notice begin a little before a batch of new castings were received from a casting company.
Castings are ordered, several thousand at a time. As they are used and the number on hand dwindles, the maker will order another batch of castings.
In the jet engine world, we called each of these batches of castings a "lot".
It's fairly easy to determine when a "lot" of castings arrive and went into production. It is more difficult to determine when the last castings from a "lot" are totally used up and often there is a intermixing of older and newer casting lots towards the end of production.
For that reason, in an attempt to get all of the problem castings, usually the ending date or last serial number selected for a recall greatly overlaps the time when the last casting has found its way into the production cycle.
In this case, it does look like a casting problem.
There can be any number of reasons something like this can happen.
The casting supplier might have used the wrong alloy, the wrong pouring temperature, or changed his procedure.
Another possibility is a new casting company may have bid the job and been selected to produce these parts.
Often, different casting companies will change the way the patterns are gated to feed the molten metal into the mold cavity and this can effect the final quality.
I think Lyman and Interarms (who must have agreed to the recall) did right in issuing the recall.
It's only too bad they didn't get the word out as effectively as some of the companies that sell the guns.