If it is a 1968 model year-produced Atlas, the frame number would be somewhere between around 124300 and 126145. Most of the 1968 production after this number were Commandos.
If the number is earlier, it is actually a 1967 model year bike.
Here in the US, bikes were often titled in the year they were sold, rather than the model year the factory produced them.
Atlas bikes produced after 121000 (mid-year 1967) would have a distributor and Amal concentric carbs. Bikes built before then would have a magneto and monoblocs.
'73 Norton 850 Commando - 3030xx
'69 Norton Ranger 750 - P11/1289xx
'67 Matchless G15CS - G15CS/1235xx
'61 Matchless G80CS - 61/G80CS/41xx