We see in both S1 and S2 that Aziraphale keeps Crowley's 1941 hat on the coat rack of his shop in the present day. (Yes, I know it's Terry's hat but indulge me for a moment.)
In the S2 1941 minisode, Crowley was wearing it when they were back in the shop for their candlelight wine date.
So when did he remove his hat?
Maybe he rushed out after a rather intimate moment and left it behind accidentally. Or perhaps Crowley left it deliberately for his angel. It's a little reminder of that certain night and a glimpse of what domestic life could be like for them. And Aziraphale keeps it, chooses to leave it out in the bookshop where he sees it all the time. His memento from 1941 is Crowley's hat, and Crowley's memento is the eagle lectern from the church.
I know we all lost our minds when we saw the eagle lectern in Crowley's flat, where he walks past it regularly, a constant reminder. But I think Aziraphale's keepsake in the bookshop is absolutely sweet and perfect for him. (And however the hat ended up there, Crowley did not get it back!)