Have an older day n night gas/ elec roof pack about 18 years old, single phase, 4-ton, need the model number.

The model number is located on a permanent tag on the unit. There could have been more than 150 different models made of that unit over the years.

Bought a house, Furnace runs fine. Went to change the filter and there isn't one I can see. Day & Night furnace. Where should the filter be?