Wheel Size Chart

I have an old bike which I dearly love. It has 27 tires on it and I use 27 x 1 1/8 - 1 1/4 tubes. I have a Sigma 1606L computer which can be programmed for two bikes. I am currently waiting for the second wheel rear wheel kit to arrive so that I can mount it on my beloved "little blue bike."

Here is the question, when reading the wheel size chart for the Sigma computer it list a 27 x 1 1/4 and a 27 x 1 1/4 Fifty - what is a Fifty?
I am assuming this is what is on my bike given when I physically measure the tire as shown in the owner's manual and calculate the wheel size I arrive at the same number for the "fifty." Basically, I'm just curious what this means.