The System 4 computers were a very different architecture to ICL’s mainstream 1900 series. The former were byte-based, whereas the latter were 24-bit word based (from my failing memory). I tried, reasonably successfully, to avoid the 1900 series. The System 4 was eventually phased out, another example of what certainly seemed to be a superior technology being ousted by a more popular one.
To get to the point, the paper tape reader at the Post Office was ‘A Beast’, both in terms of trying to keep it working, but especially in terms of performance. This thing could read around 1400 characters per second and if you don’t think that’s impressive, build something that literally shoots paper tape one metre horizontally before it even has time to droop.
Paper tape is about an inch wide and comes on spools up to 8 inches or so across. It has a sprocket hole down the centre (roughly), and 9 hole positions across the tape, for one byte per position, plus parity. There was longitudinal parity too and I think also a check sum - these things were seriously error-prone, especially at high speeds. (looking elsewhere on the Web, I see 5 holes per tape was the rule. When I get a minute, I’ll go find the unit shown here and have a closer look).
To read a tape, it was first loaded on the reader supply spool, with the tape threaded over some spring-loaded take-up arms, then over the sprocket feed mechanism, though some more take-up arms, then onto the take up spool. The two spools were servo-driven from the take-up arms, like a magnetic tape drive. That worked OK on slower paper tape readers, but from memory again the one at the Post Office was so fast the take-up spool couldn’t keep up, so it had a big hopper where the tape could drop - hopefully without tangling. That might also have been an option for where the tape was to be re-used. Regardless, the controlling computer issued a read request, which set the tape moving past the light-sensitive heads. As each character went past it was buffered and when a block was complete it was error-tested and sent on to the computer, which did what it needed to do. The tape could accelerate and decelerate extraordinarily quickly, much faster that the eye could follow. Roughly speaking, it had to be able to stop in a single character distance (which isn’t much at 1400 cps).