The OTL 100,000 professional army a with service term of 25 years ...
That was the maximum service length for men other than generals or marshals. I heard Kleine-Albrandt lecture on what he learned from interviewing surviving Reichswehr officers from the 1920s. I can't recall what the initial service contract was but the turn over was fairly high. Only the top men were expected to stay on. Those discharged were encouraged to participate in the hidden reserve. The bulk of this hidden reserve were in "Police Auxiliaries" or police reserves. That gave them organization and small arms including MG. If my lecture notes are correct there were close to 300,000 men in these police auxiliaries or reserves. Part were recently discharged Reichwehr men & part Great War veterans. Another portion were employed as civilians in the administration dept for the Army. Those were doing the same jobs that uniformed quartermasters and other staff did in other armies. Gun clubs & some veterans organizations had some groups considered reserves by the Reichwehr, tho Kline-Albrandts informants indicated the numbers for that group were exaggerated. Also exaggerated were pools of hidden equipment. The Police organizations had a large amount of small arms and ammunition, but hidden reserves of artillery, transport, communications equipment, and especially ammunition were a lot less than claimed. Kleine-Albrandt stated about 500,000 trained and armed men could have been mobilized by the Reichwehr, but less than 200,000 could have been in fully armed infantry divisions. The balance would have been in battalions, regiments, ect... without significant artillery or transportation.