In On Utumi's ring of quotients, Canad. J. Math. 15(1963), 363-370, J. Lambek says:
As a matter of historical record, the minimal injective extension of a module is a special case of the "algebraic closure" of an algebraic system considered by K. Shoda in his paper Zur Théorie der algebraischen Erweiterungen, Osaka Math. J., 4 (1952), 133-144.
Since I cannot read German, I cannot understand what the paper by Shoda says, although I think that he does defer the definitions to his previous papers Uber die allgemeinen algebraischen Systeme I-VIII (links provided below).
I would like to know:
What exactly his algebraic systems are (maybe the varieties of universal algebra?).
Which are his fundamental results about said systems (in these or other papers).
A published translation/review/summary would serve me perfectly. The MathSciNet reviews of the first papers are too vague, enumerating examples and talking about "certain operations", "various relations", etc.
Links to the I-VIII papers:
https://projecteuclid.org/download/pdf_1/euclid.pja/1195578672
https://projecteuclid.org/download/pdf_1/euclid.pja/1195573978
https://projecteuclid.org/download/pdf_1/euclid.pja/1195573940
https://projecteuclid.org/download/pdf_1/euclid.pja/1195573901
https://projecteuclid.org/download/pdf_1/euclid.pja/1195573626
https://projecteuclid.org/download/pdf_1/euclid.pja/1195573484
https://www.jstage.jst.go.jp/article/pjab1912/19/9/19_9_515/_pdf
https://www.jstage.jst.go.jp/article/pjab1912/20/8/20_8_584/_pdf