The number on the sticker in your photo is the model of the tube, and the tube alone. From Atomised you'll note it has a CR-31 neck, which is the 8-pin variety and obvious when you look at it, but it also has a 6.3V heater. To use the same chassis as is, you need a tube that matches those two elements, but also one with the same resistances across the yoke.
The chassis will have a separate model no. on it, and it will be different to the tube number. It might be made by Toshiba as they made both tubes and chassis, or it might not. And this monitor (combination of tube and chassis) probably had it's own model no. to reflect this combination of tube and chassis. Added to which this may have all been from the '80's, and a lot of this stuff was never written down properly.
Also, the two starting digital of the tube model being 51 might indicate it's a 51cm/20" tube, not a 48cm/19". Are the corners of the screen's visible area nice right angles, where the phosphor stripes end, or curved with the radius of a ten cent coin? If the latter, then it is a 48cm tube.