Definitely pre-1960 ... (60 - 62 pre-unit bikes have a duplex frame) Looking at the Rask serial number link in the previous post lead me most conclusively to say that it's a '57. Model number is in the engine number (T110).
Check the frame (left side near headstock) and see what the number is on it. The serial should match the engine.
It certainly is a great find and seems mostly there. You have a lot more of it than I have of my 57 T110 (which is not a numbers bike ... it's more of a "50s Bitsa Pile").
I saw that at first, too, Paul ... it appears to be an optical illusion ... shadow from the camera flash. You can clearly see the angled tube from headstock to where it turns down and then the horizontal 'seat support' rear frame section sticking out behind it. The frame looks like a stock late 50s Triumph frame to me. In fact, from what I can see of it, it looks just like my 57 frame.
The numbers will be just in front of and possibly slightly below the fuel tank on the left side of the front frame tube. Sometimes they're somewhat 'filled in' with paint, etc. Go at it with a wire brush or abrasive pad and it should make itself seen ... unless you don't want to damage the finish on the frame.
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