Friedrichs' early contributions are discussed in <A HREF="https://link.springer.com/chapter/10.1007/978-94-017-2012-0_9">On the Stone-von Neumann Uniqueness Theorem and Its Ramifications</A> by S.J. Summers:

> In the early 1950's, K.O. Friedrichs undertook an  influential attempt
> to reduce the hand-waving typical of quantum field theory up to that
> time [1,2]. The result of greatest interest was his construction of
> some representations of the canonical commutation relations for
> infinitely many degrees of freedom which were *not* unitarily
> equivalent to the Fock representation.

[1] K.O. Friedrichs, Mathematical Aspects of the Quantum  Theory of Fields, 1953: page 3.     
[2] see also Commun. Pure Appl. Math., **4**, 161–224 (1951); 
**5**, 1–56 (1952); **6**, 1–72 (1953).