Furnace filter conventions

I posted something similar in alt.home.repair but I wanted to expand on it a little in the more appropriate group.
When looking at my home furnace I'm struck by just how little is on it. Aside from the manufacturer (Payne) and a certificate indicating when it was installed (April 1983) there's absolutely nothing on it - no model number, no filter sizing, no anything that would be of value to a new homeowner.
First, on the inside of the filter cover there are wiring and mechanical schematics indicating possible model numbers but nothing conclusive. In my case I spotted 375A and 397H but nothing to tie it with a model number. Only my experience with engineering drawings let me to think that was the model numbers appropriate to the schematic.
So I'm trying to replace the filter, the current one that was sold with the house is this green mat of plastic fibers and is totally impregnated with dust. Unfortunately it has only rough dimensions and works out to be 14x27" So I played around with different cheap frame filters and discovered that a 14x24 fits in the filter slot VERY tightly and seems pretty deformed at that. A 13x23 would fit perfectly.
So my real question, is there a clearinghouse that I can go to for correct filter sizing? How can I get a model number on this furnace (gas) without dismantling it?
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Neither is the furnace, its long past its normal design service life

Still doesn't matter

14x24 is a standard size and readily available from most suppliers.

Why not just have a filter base installed when you have your new furnace installed. FWIW, they only build them to last 18 - 20 years. Yours is already way past that.
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So the answer is "No" there are no clearinghouses or conventions used for gas or electric furnaces? How would a repair technician know what size filter to bring? Blind luck?
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Legitmate contractors are prohibited from installing used fossel fuel appliances not only by mechanical, gas, and fire codes, but insurance companies take a dim view of it too. The best you can hope for with a used furnace is maybe a buck or buck and a half at the scrap yard.
As I said before, 14x24 is a standard size, so is a 14x25 furnace filter.
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You could try this, http://www.filterclearinghouse.com
HTH, -zero
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