The trans-atlantic crossing was Shannon to New Foundland. The route was Southampton to Baltimore.
In 1939, PAN AM started two weekly routes to Europe. One was a passenger flight on the southern route via the Azores on 30 March, with a passenger service added 28 June. The article does not say when it returned. However, your route is the northern one and cannot be this.
The second route was the route shown on your cover. This left for the British Isles on 24 June 1939. It was a mail-only service. The passenger service started 8 July. Your covers must have been a return flight on the northern route before the passenger service was added.
The westbound flight left on Saturday morning (24 June, 8 July), arriving Sunday. They returned from Southampton on Wednesday (28 June, 12 July).
Your first cover may have been carried on the first westbound mail flight.
The sender, probably, intended the second cover to be flown on that flight and returned to him (hence the c/o Pan Am that was struck), but did not post it in time and then used it as a 'normal' cover. Another explanation was that it was intended to be carried on the first passenger service on 12 July 1939. However, that seems questionable. It would have made the 5 July mail flight.
See enclosed links. For Wikipedia, search 'Yankee Clipper.'https://www.panam.org/explorations/...enger-flighthttps://en.wikipedia.org/wiki/Pan_Am