I think it’s from a batch of outlier 321 movements. Perhaps @Dash1 can shed some light.
I have a 27.320.*** cal.321 in a 145.012-68 but I never got round to ordering the extract (been high on my to do list for at least a year, unfortunately my to do list would take 3 years to clear up!)
My understanding is that this range is only fitted to the 145.012-68 originally but most of this range are in fact service replacement movements. My money would be on a service movement in the case of the OP’s example, but wouldn’t it be great to be proved wrong!