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.

This development is described by Friedrichs on page 3 of [1]:

> In Part IV we shall show that a description of the field in terms of
> "occupation numbers", though somewhat unwieldy, can nevertheless be
> introduced, even if the spectrum of the energy is not discrete. It
> will be seen that there are two types of such occupation number
> representations; only one of them is equivalent with a particle
> representation. Accordingly, there are different non-equivalent
> realizations of the basic field operators, and consequently different - non-equivalent kinds of fields, a fact which seems worth noticing.


[1] K.O. Friedrichs, <A HREF="https://archive.org/details/mathematicalaspe0000kofr/page/2/mode/2up">Mathematical Aspects of the Quantum  Theory of Fields</A>, 1953.     
[2] The 1953 monograph, in four parts, is based on four earlier publications in Commun. Pure Appl. Math.: **4**, 161–224 (1951); 
**5**, 1–56 (1952); **5**, 349–494 (1952); **6**, 1–72 (1953). 

So the "late 1940's" from Reed & Simon should more accurately be "early 1950's".