Two dips in the transmission spectrum of a 1550 nm fibre Bragg grating have been observed at 2/3 of the Bragg wavelength (λ2/3B, i.e. near 1030 nm) in a Bragg grating fabricated in non-birefringent single mode fibre using phase mask technology. It has been proposed that these features are due to the complex refractive index structure generated during the fabrication process. In non-birefringent fibre, transverse strain causes the Bragg wavelength peak to split due to the induced birefringence and we expect a similar phenomenon to happen at λ2/3B. Only one of the dips at λ2/3B became split and the separation increased linearly with transverse strain, whilst the other dip did not exhibit any splitting. The dip separation per change in applied load at 1030 nm was 2.3 pm/N, which is consistent with the dip separation per change in applied load at 1550 nm (3.8 pm/N), suggesting that the cause is induced birefringence.