Age of a Ducane Furnace?

Can someone help me figure out the age of a Ducane furnace and how you determine it? The model # is MPGA100848. Serial # 8503740637. Thanks.

That one should be January of 1985. The first two digits being the year, and the next two digits being the week.

How about this one?
Model Number MPGA075B3
S/N. 6233530020
Second block below s/n: 20251802B

Ducane**uses two different formats on serial numbers. All of their serial numbers are ten characters. The most common format is all ten characters are numeric. In this format the numbers in position 7 and 8 indicate the calendar year and the numbers in position 9 and 10 indicate the week of the year. An example is XXXXXX9933 where the 99 would be 1999 and the 33 would be the 33rd week of the year (1999). The more recent format is ten characters. The format is that the first four characters are numeric, the fifth character is alpha, and the last five characters are numeric. The year code is determined from the numbers in the 3rd and 4th position, which would be the last 2 digits of the calendar year (ex: 06=2006). The fifth character which is alpha represents the month with A=January and M=December and but they do not use the letter I.