Probably depends on what other plans the manufacturer has. Not speaking specifically about the 121's, but in general.
If you plan launching rolling stock, that will sell better if you still have locos left in stock. So, you could do a small batch of locos and a small batch of rolling stock, and not bother with the two batches aiding each others sales, or you could do larger batches of each, where the sales of each boosts sales of the other.
Like Guinness and toilet paper side by side in the supermarket.