I concur with Joe.
The only number that tells me anything is the Engine number. The first two digits tell me its a 1972 model but not the engine size. It could be a 10 HP engine but, if it is original to the tractor it will be a 12, 14 or 16 HP as the LGT-100 (10 HP version) was not available with hydro. Unless you can find some remnants of a designation on the shroud, I don't know of any easy way of positively determining the HP of the engine. There should be a decal located on the air cooling shroud either on the carburetor side at about the level of the points cover or on the flywheel side near the top. The designations are as follows: K241=10HP, K301=12HP, K321=14HP and K341=16HP. The model number is cast into the block but is not visible without removing the electric PTO clutch. These are wonderful little tractors and I have 10HP, 12HP and 14HP open side versions as well as two 16HP closed side versions. Only the LGT-125 is currently running. I have the engines for a 14HP and a 16HP bored over size with everything I need to completely rebuild them except the gumption. Good mower decks are becoming difficult to find for these machines. I have a 48" blade on my LGT-125 and it can push a lot of snow for a machine of its size!