The marks on the speedometer indicate the suggested maximum speed in the various gears. You can go a bit higher, but then the rev-limiter in the fuel pump relay shuts the fuel off momentarily. The "I" mark is for first gear (even though your shifter doesn't have a "first gear" marking since it is only actuated from the kick-down switch). The "II" mark is for second gear...and so on. They don't give one for fourth gear (though some of the old diesels had a "maximum" mark)...I guess it's because at top speed your attention had best be on the road. The marks are also handy for knowing when it's okay to down-shift to a lower gear without blowing a chunk out of the block.
Randy D.