Alpa factory updates?

I've got an Alpa 6 that, based on the serial number, was manufactured

in 1958 as a "6". However, it's got the lever film transport and

instant return mirror of the "6b", although without the diagonal

split rangefinder.

Everything about the camera looks genuine, so I'm guessing it was

returned to the Alpa factory at some subsequent date for an update.

Can anyone confirm if Alpa actually offered an update service?

Ivor Matanle in his book "Classic SLRs" wrote "Unlike other manufacturers Pignons did not cease making the knob wind models and it is by no means uncommon to find an Alpa 5 with a number later than that of a 5b. In fact Pignon's policy throughout was to make what the customer wanted and it remained true ...in the late seventies that Alpa would make an Alpa 7 to order, albeit at a price." With this attitude to the customer I imagine they were willing to update a customer's camera.

All best

Keith H-L