if you mean the brass bar located on the pillar right above the serial number and below the Badge - that is the model number plate. You don't find that on earlier Singers. It should say 15J, or possibly 15-J-88 . J donotes made in Canada, K denotes made in England, no letter is USA, until they started using A. It actually denotes the location of the Plant.
BTW - for this series model 15, 88 is treadle, 89 is hand crank, 90 is external motor, 91 is potted motor. The 88 and 89 will have the larger spoked handwheel, but other than that, they are the same machine as the 90. I had a nice 15-88 in a horrid treadle cabinet. Took it out and put a motor on it and it's one of my favorite machines.