I have a 1950s book on rugby that describes both formations.
The 1952 RFU Manual of Rugby Union Football? (The first the RFU produced.)
"
The 3-4-1 formation has now become the general practice infirst-class football" (page 60)
It later gives some comments on the relative merits of the formations.
3-2-3 or 3-4-1?
26. Among present-day players opinion is swinging more and more in favour of the 3-4-1 scrum, for the following reasons:
(a) Four men can apply their weight directly to the front row, leaving the two locks to take the weight of the No. 8 only.
(b) The inward shove of the wing forwards on the front row holds it together, counteracting instead of reinforcing the tendency of the locks to split it apart.
(c) The shove is concentrated on the opposing hooker.
(d) The ball has to come back through two rows only, and with a properly timed shove and good hooking should come straight out on the scrum half's side of the No. 8.
(e) The shove of the more compact 3-4-1 body is better concerted. The formation is designed to produce a quick "heave", to carry the hooker over the ball at the ideal instant, rather than a sustained pressure; there should be no need to walk two or three paces forward to let the ball out, as is often the case with 3-2-3.
(f) The wing forwards are a pace nearer the advantage line and to that extent better placed to spoil the opposing scrum half or get up on the fly half. Moreover they are better able to watch the progress of the ball in the opposing scrum and thus to time their break correctly.
(g) The wide second row forms a good protective screen for the scrum half when he is getting the ball away.
(h) The No. 8 can make a quicker break, as he has no one binding on him.
27. As against this, the possible disadvantages of 3-4-1 are:
(a) The formation is perhaps less well adapted to "holding" the ball in the scrum, and is certainly unsuited for wheeling. In fact, when a wheel is to be attempted it is best to drop the wing forwards back temporarily to the third row.
(b) An inexperienced or ill-balanced 3-4-1 scrum can be a very ragged affair, much given to slewing.
(c) The timing of the shove is by no means easy.
(d) The "forward" position of the wing forwards and the comparative freedom of movement of the No. 8 may tend to over-develop spoiling tactics at the expense of shoving.
28. Every team must decide for itself which formation to adopt, and the choice may well depend on the size and physique of the players in the pack-remembering in particular that two forwards with very strong backs are essential in a 3-2-3 scrum if the thrust of the back row is to be transmitted with full effect. Each formation has its own special technique, which must be learnt; it is quite fatal to suppose that a 3-4-1 scrum can be formed simply by moving the wing forwards up one row.