Sisu S-21 was developed during the Continuation War for the Finnish Defence Forces to be produced in relatively large quantities.

VAT replaced the model by 1951 presented Vanaja VK-series; in the meantime, SAT developed its follower named Sisu K-23.

[7] During the Continuation War in 1942 Finnish Defence Forces estimated that it will need 7,000 lorries and buses within the following years.

SAT, the only domestic vehicle producer, did not have enough of technical and economical resources for a such large scale production, although the company was building a new facility in Karis.

Nessling, who was also the main owner of SAT, ruled out the offer of the state becoming a co-owner in the company.

SAT committed to hand over to Yhteissisu the needed drawings, other production documents and list of subsuppliers.

In the meantime, SAT was not allowed to produce lorries, apart from the 120 units on its order log.

The first batch of 300 units was scheduled to 1944, after which the annual quantity was to be increased to 700, and the remaining 1,000 were to be delivered by the end of 1946.

[11] According to the contract the production had to be started directly, and as no other facilities were available, the first Yhteissisu vehicles were built under a separate bookkeeping at SAT works in Fleming street, Helsinki.

[12] Yhteissisu finally found a suitable area for vehicle production in municipality of Vanaja, next to Hämeenlinna, where it built a facility of 25,000 m2.

[11] The first serial produced vehicle was scheduled for 8 November and it was prepared to be a showy media event, but due to material shortage, lack of workforce and quality issues the serial production could not be started before early 1946.

[14] At the beginning Yhteissisu production was dependent on availability of German components; gearboxes and steering gears were from ZF, rims from Kronprinz, cardan shafts from Rheinmetall and Bosch supplied electrical equipment.

Valtion lentokonetehdas made cabins and cardan shafts, Valtion Tykkitehdas steering gears, rear axle housings and wheel hubs came from Ahlström, drive shafts from Lokomo and rubber parts, including tyres, from Suomen Gummitehdas.

What is more important, the war was over and the Defence Forces had to reduce the number of vehicles based on the peace treaty, so buying new ones was out of question.

However, ten semi-finished vehicles were delivered as Sisu's, as the customers had ordered them such, after a special permit from SAT.

The number of produced Sisu S-22 and S-22K totalled 294 pieces, which was way less than the originally planned 7,000 units.

The total quantity was then 650 units[17] and the state gave a significant prepayment of 132 million marks.

[17] The company General Manager Eero Kytölä, travelled twice to Germany and France for component purchasing in 1947 and 1948.

He made an agreement of buying a large amount of surplus material of Western Allies.