To add to this -- the dash numbers on the 5101s are indicative of the speed. But, many of the 5101's date back to late 1970s and the correlation between dash number and the speed was based on phase of moon, flip of a coin and how many pennies were in the jar. Nearly no rhyme or reason as to how they were numbered. You need to find the data sheet for the specific brand that you have to cross it over to speed.

Phillips was the last manufacturer for 5101's. They were cranking out PCD5101P's by the gazillions at the end. They fast, low power and dirt cheap - nobody else could compete with them. But, alas, demand for these died out with flash memories and NVRAMs and they went the way of the DoDo bird. These PCD5101Ps were the fastest of the 5101s at 150nS and were amongst the lowest in standby current at about 0.02uA when in battery backup mode.