Larry, the Roper 20T was only made in 1979 so that looks like what you have, they quit making this body style after that.
Manual can be found on this site, just have to scroll down left side, pick Roper tractors, then the 20T model.
Got this off another site so I cannot verify info or links....
RRoper 42" deck Model #TT2131R
Roper 42" deck Model #TT728CR
Roper 48" deck Model #TT738CR
Sears 42" deck Model #917.253250
Sears 42" deck Model #917.253540
Sears 42" deck Model #917.553580
Sears 42" deck Model #917.253582
Sears 48" deck Model #917.253551
Sears 48" deck Model #917.253590
Sears 48" deck Model #917.253592
& Possibly Others.
Bearings & Races
Outer Mandrels, )you will need 2 for each mandrel, 4 in total for the deck.)
Outer Mandrel Bearing & Race, Sears Part #1554H & 1553H, use Napa part # SME 704298
Metallic Grease Seal, Sears part #881R. this part is available from Sears Parts Direct.
Center Mandrel, (you will need 2 for each mandrel, 2 in total for the deck.)
Center Mandrel Bearing, Sears Part #888R Use Timken Roller Bearing Part Number LM12749
Center Mandrel Race ( Cup), Sears Part #9494H, use Timken Roller Bearing part # LM12711
Metallic Grease Seal, Sears part #882R is no longer available, however it appears that part #881R can be used, but the center hole will need to be enlarged from 0.750" to 0.8658" it will then fit the shaft. ( I have not actually tried this, however using the bore diameter of the bearing, this measurement would be correct for the seal to fit over the shaft). On a complete deck overhaul, this would bring the required number of the 881R seals up to 6
The deck belts ( Center Mandrel to outer) for these decks are:
For the 42" decks, the original sears part number is 6942R, which is a 1/2" X 87" belt. This can be replaced easily by using Gates Belts part # A85 or by using Napa part #NBH A85.
For the 48" decks, the original sears part number is 6943R, which is a 1/2" X 92" Belt. This can be replaced easily by using Gates Belts part # A90 or by using NAPA part # NBH A90.
The Drive Belt ( Engine to center mandrel)
The Engine to drive belt listed in the manuals for the above mentioned decks is either sears part #6941R or sears part #3555R, both of which are 5/8" X 103" & they both interchange to a Gates BX100 or NAPA Part # NBH BX100 This assumes that you have the correct mule drive that came with the deck. Older mule drives had smaller diameter pulleys, if yours is not the original one for the deck, you may need to try a 102" or even a 101" belt to accommodate the smaller pulley diameter.
The 42" decks need 3 - 14 5/8" blades, the original sears part number was either 9480R or 9484H, both of which interchange to NAPA Part # SME 704523. This blade can also be purchased online thru www.findparts.com
The 48" decks need 3 - 16 5/8" blades, the original sears part number was either 9485H or 9497R, both of which interchange to NAPA Part # SME 704524. This blade can also be purchased HERE