#ogagiri

my timeline for "Joker Game" might help you.

Anyway D Agency was founded in 1937, autumn. Around that same time Sakuma was ordered to become the liaison but, before going there he had to wait for his hair to grow longer so he went there in 1938 April and witnessed the exams of the various spies until only eight of them remained.

Well, according to the novels from which the anime is based:

"The training in the D-Agency was full of diversity.

For example, the students had been instructed on using explosives and wireless signals, the operation of automobiles and planes, as well as learning various languages and dialects. They had received lectures from professors of renowned universities on organizational theories, religious studies and international political theories. Lessons on medical science, pharmacy, psychology, physics, chemistry and biology were also given.  

Abstruse discussions about ideologists and strategists such as Sun Tzu, Kant, Hegel, Clausewitz, Hobbes, and more that Sakuma hadn’t even heard of, often happened between students. On the other hand, professional thieves and safe-crackers had been brought out of prison to instruct the trainees on the techniques and methods. Aside from being taught on how to pick a lock using a single wire, the students also received lessons on how to secretly switch items by sleight of hand as a magician would, dance and billiards techniques. Kabuki actresses had been hired to teach the students the art of disguise, while professional gigolos demonstrated how to seduce women.

All students had been required to swim in cold water while clothed, then travel to a location far away without sleep, and use complex ciphers, memorized only a day beforehand, fluently and naturally as the languages they usually speak.

The D-Agency had even trained them to be able to disassemble shortwave radios in complete darkness, and put them back together so that they were once again functional, relying solely on their fingertips. The students had also been ordered to seamlessly open envelopes with a bamboo stick, and instantly memorize words which had been written backwards and reflected in a mirror. Letters containing orders, no matter how complicated they are, were to be destroyed on the spot after they had been read; the students had also been trained to recover destroyed letters." [Joker Game Bool 1: "Joker Game" translation courtesy of @imayooshi]

Also Yuuki himself taught to the students as well.

The image you saw is an art from Shiro Miwa, "joker Game" character designer. It reflects his own headcanon about the future of the characters from "Joker Game".

Shiro Miwa said he assumed that, after the war Yuuki forced Kaminaga to take over the organization and disappeared with Jitsui and that Kaminaga kept on receiving instruction from an unknown address. He also said he picked up 1960 because it was a time of major changes in world affairs including the Vietnam war and Kennedy assassination.

Amari apparently ended up on a wheelchair and is being taken care of by Emma.

According to Shiro Miwa the one I speculated is Miyoshi (at the time I didn't have the book with the illustration and so I couldn't read what he wrote as that image was shared by @i-dedicate-this-kill-to-the-fans here) is someone else who got recruited by Yuuki while he was in hiding.

However this is just Shiro Miwa's headcanon. Due to lack of material many fans tend to accept it as canon but it's not really canon and if the series were to be continued in the future things could go differently (especially since Odagiri and, likely, Sakuma were both likely set up to die in war).