Mine is clearly visible on my TSR 27 but almost impossible to read on my Blue TSR 30. I only know the number on the latter because it’s been documented.
Both are located where Ian says. The best position to try and see it, unless you separate, is to knell on the left-hand side of the bike, shine a strong light slightly upwards to highlight the back side and underside of the frame spar where the bolt goes through.
May be possible to do a “brass rubbing” by sticking a piece of tracing paper where the numbers seem to be and rubbing with a pencil and then holding to a mirror.
Alex
Alex