Hi Freeman, I can explain/answer:
The Seacheros (and Railmaster, both 135.004 and the more collectible 2914) have PAF because they were issued to the Pakistan Air Force in the early 60's. Hence they are regarded as military watches as well.
Both the Ranchero/Seachero and 2914 Railmaster also have the PAF engraving on the movement, although the 135.004 Railmaster PAF does not. There is speculation (unverified) that the Railmaster and Ranchero's were both issued to PAF at about the same time (July/Aug 1960 onwards) and if you do a reference check of the serial nos (for which I maintain an Excel sheet) the serial no range of these 2 references are very much in the same bracket.
The Ranchero was a different watch than the railmaster. Ranchero was 36mm while Railmasters was 38mm. They have similar aesthetics but this is true of all vintage Omega professional lines from that period (see also the Seamaster 2913 and of course the Speedy 2915, all of which came out in 1957 (except for the Ranchero which I believe came out later).
Curiously, while the PAF commissioned these watches, they did not want either Railmaster or Ranchero on the dial (which is how they were sold in their non-military versions). For this reason, the dials of these read Seamaster. Why? No one knows but this is what PAF wanted and Omega complied.
Speedmaster, even Flightmaster might have made more sense. But apparently, there was some negative connotation associated with Rails in Pakistan (it is seen as lower class mass travel and their conditions to this date are rather poor, plus the memories of the time of British colonialism when these railways were introduced also probably had something to do with it).
The hands are different - Seachero has "dagger" hands, while Railmaster (2914 at least) has broad arrow hands. The 135.004 Railmaster (issued in 1964) has sword hands.
I think the movements of both Ranchero and Railmaster are the same but am not sure. However, the casing for the Railmaster is different (it has an additional inner case back to act as a shield from magnetism, sort of like the Rolex Milgauss).
The rest, as they say, is...history (or speculation).