According to the I&T manuals; Champion 15A
AC 85S
Auto Lite BT8
I'm sure these numbers are outdated, although you may find some "vintage" plugs on ebay. In mine, I have D15Y's, not even sure that number is any good any more.
Below is copied and pasted from NAPA online, crossing the 15A number, and got an Autolite ASP 386.
Spark Plug - Copper Core
Product Line: Autolite Spark Plugs
Interchange Mfg.: Champion Spark Plugs
Interchange Mfg. Part Number: 15A
Compare ASP 386 This part can only be purchased from your local NAPA Auto Parts store
Here is the link to NAPA online
NAPA Online
In the menu on the left, select advanced search in yellow.
Then select specific vehicle on the next page.
Next page it will show vehicle years. Up in the righthand corner, it will say "change", in yellow, click on it.
On the next page, find Tractors/Agricultural, and click on it.
Next page, select your year.
Next page, select vehicle make.
Next page, select vehicle model.
Next page, type in spark plug, then hit search.
You should get 4 results... 2 Autolites, and 2 Champions.
This search is handy for finding bearings too... But from the beginning, when it asks if you are looking for a specific vehicle, click on "No", then type in a maunfacturer's part number. Then at the bottom, be sure and click on the interchange. There should be a window that comes up to give you the option to find a local store, or buy online. Click "buy online". It will usually show the bearing, and like the Timken, or universal number for that size bearing, and price.
I've found a ton of bearing numbers that way, then Google that number, or searched on ebay for a particular bearing. Usually you can buy them super cheap on there. I replaced all the front wheel bearings w/cups on my Massey 180, inners, and outers for less than $20.00, including shipping. That's less than most want for a complete set, for one side..., and usually China bearings... There are many NOS U.S.A. Timkens on there, if you know your number.
Sorry this is so long, but if it helps someone find a part by cross reference, it was worth it...
Hope this helps..!!