Peter, it's being assumed here that production of helmets continued or started up soon after the war while huge stockpiles of helmets in depots all over Germany were being sold off or scrapped. This was not the case. The distinction between civilian and military models must also be made clear.
Officially, Germany had no plans to manufacture steel helmets in the years following the war. I could find no evidence that large scale production of helmets for the military occurred in the 20s. What was also clearly stated was that when production did start up again, the plan was that a new streamlined and lighter helmet model would be developed first. A few upgrades and changes were made to liners and chinstraps prior to the introduction of the M31 liner system.
Large amounts of war material had been lost by war's end, including helmets. In addition, various Freikorps, Einwohnerwehren and Red Brigades were using them. According to records, many depots were found to be mostly empty of helmets, and it was feared that there would be shortages for the post-war army.
As an example, when around 500 helmets were ordered from the depot in Ingolstadt in 1919, it was found that only 350 German helmets remained with an additional 60 000 captured British and French helmets in stock. It was even debated for a time whether or not foreign helmets should be issued.
The Armee-Verwaltungs-Departement is on record offering to buy steel helmet shells for 14.50 Marks and complete helmets for 19.75 in 1919. (This is clearly stated in Ludwig Baer's German language edition "Vom Stahlhelm zum Gefechtshelm" printed in the 90s. His English edition "History of the German Helmet" from the 80's states the exact opposite, claiming that helmets were being sold for those prices by the ministry at this point and that demobilization had led to vast amounts of material and helmets being available (made surplus) - as opposed to having been lost (verschwunden), as stated in the German text. This is a mistranslation.)
It was around this time that the Armed Forces lay claim to all remaining usable helmets found in the various helmet manufacturing plants.
Remember, this is prior to the limitations set by the Versailles treaty in 1921.
Occasionally, examples of M18 style transitional helmets (droop bill visor, etc) turn up that are clearly post-war manufactured. These and other civilian models, like the one in this thread, were likely manufactured a decade or more after WW1 had ended. German fire departments mostly still used leather helmets up until the late 20's, and light-weight civilian models only started appearing around then in any numbers.
I think it's obvious that once the improved M35 started production, the old helmets were slowly phased out. Some old models were reissued to German soldiers during WW2 to make up for temporary shortages.
As to exported helmets, aside from those issued to Bulgaria and Austria during WW1, it seems that most countries like Finland received large shipments of WW1 era German and Austrian helmets only during the late 30's, well after the introduction of the M35. In fact, many M35s were also exported. There is no record I can find as to when the WW1 era helmets were shipped to Afghanistan.
~Hans