Hi, you can try renaming [PostingMonth of your production table to [CreateMonth], so after the join you end with a row that has ID, CreateMonth, Production and Sell.
The stock is Production-Sell, but it needs to accumulate from the first date, and probably you'll need a initial stock for each of your products, but for this adte, the accumulation can be done in the Calculate Table as:
RangeSum(Peek(Stock),Production,-Sell) as Stock
Order By CreateMonth;
DROP Table Production;
DROP Table Sell;
Note that I kept production and sell in this final tabla nd remove the previous table, so only the Calculate table i kept in the final data model. I also remove the ID =1234, I didn't know what are you trying to do in that syntax, if you want to apply a filter for a product you can use the Where clause (Resident production Where Id=1234...)