A new paper tells the story:
As we approach the centenary of the discovery of quantum statistics in 1924, it is important to revisit Bose’s original derivation of Planck’s law usually ignored in most standard presentations of Bose-Einstein statistics. It introduced not only the novel concept of the indistinguishability of photons but also of their intrinsic spin, a fact unknown to most physicists. ...Bose not only derived the Planck law using the statistics, he descovered spin to account for that factor of 2. He later explained:
On June 4, 1924 he sent a short paper ‘Planck’s Law and the Light-Quantum Hypothesis’ to Albert Einstein with the humble request, ‘You will see that I have tried to deduce the co- efficient 8πν2/c3 in Planck’s law independent of the classical electrodynamics, only assuming that the ultimate elementary regions in the Phase space have the content h3. I do not know sufficient German to translate the paper. If you think the paper worth publication, I shall be grateful if you arrange its publication in Zeitschrift f¨ur Physik’.
In a post card dated 2nd July, 1924 Einstein wrote to Bose, ‘Dear Colleaugue, I have translated your work ... It signifies an important step forward and I liked it very much ...... You are the first to derive the factor quantum theoretically, even though because of the polarization factor 2 not wholly rigorously. It is a beautiful step forward’
‘You know’, he said, ‘my deduction of the Planck law had a factor of 2 missing. So I proposed that it came from the fact that the photon had a spin, and that it can spin either parallel or antiparallel to its direction of motion. That would give the additional factor of 2. But the old man (meaning Einstein) crossed it out (‘budho k´et´e dil´e’ in Bengali, his mother tongue) and said it was not necessary to talk about spin, the factor of 2 comes from the two states of polarization of light.’Today we call this Bose-Einstein because Einstein translated the paper from English to German, recommended it for publication, and deleted one of the brilliant ideas.
Now we know spin and polarization are the same thing, but we say spin for particles, and polarization for waves.
Quantum spin has a long history of physicists denying that it was real. According to Wikipedia, W. Pauli introduced it in 1924 as an electron having "two-valuedness not describable classically". He first denied that it was spin, and then published a quantum spin theory in 1927.