I assume the older sets m055, m057, and m060 are no longer manufactured. If I'm wrong, please help me find them.

Dear Mr.Gomez,

every set, once released, is no longer manufactured and shall be looked for in local shops or on E-Bay.
We suggest you to check Plastic Soldier Review's Sellers list, where, hopefully, you will be able to find sets that you need.

Best regards,

Strelets

So, do you only produce the sets once until stock runs out, or do you restock?

Hola,

Sus moldes no son de metal y no resisten la produccion intensiva. Producen hasta que el molde acaba su vida util que es corta. Por eso no pueden hacer reestocajes.

Saludos

Subject to moulds' wear and tear, we can re-run sets if we see commercial potential for such re-releases. We re-released 045, M055 and M122 in the past, for example.
WWI subjects aren't particularly popular, so the chances for re-runs are pretty slim.

Best regards,

Strelets