No need to apologize for the hijack, and hope I didn't jump on you too hard about it. According to my DF150 service manual, for those boats that don't have an hour meter on the tachometer, the indication of the hours is exactly as was described to you. For three seconds, the tach will show the hours with key ON (before starting engine). Below 50 hours, nothing will show, then 500 rpm = 50 hrs, 600 rpm = 60 hrs, etc.
Then, when you get to 500 hours, it's a combination of the rev-limiter lamp flashing and the tach indication. Each flash of the lamp indicates 500 hours. So for 540 hours, the rev-limiter lamp would flash once and the tach would show 400 rpm. For 1040 hours the lamp would flash twice, and the tach would show 400 rpm, etc.
When you first turn the key on, ALL lamps should light and the caution buzzer should sound, then the hours indication takes place. So if you're not showing any flash of the rev light after that first diagnostic check, then the 500 rpm tach indication should show your 50 hours on the motor. And just as a matter of interest, even my tach, which HAS the hour meter built in, shows the same indications when the key is first turned on. The needle jumps to 1300 rpm, and I have about 130 hours on the motor.
Hope that settles your mind (and hope there's no rev-limiter light flashing!)