Autopilot upgrade

[sunnysky]

Hello All.
Im a little confused. Skywalker posts he has an Stec 55 installed with an STC ?
As far as i'm aware, there is still no STC for the upgrade of a 337 autopilot??
For the last 38 years I have owned GBFGH, a Skymaster G model.
Its got the Navomatic 400A installed which has worked brilliantly over the years.
However, now, im having issues and nobody in Europe seems able to repair it.
( for years Pete Sommers helped me , but sadly is no longer with us )
Im certainly prepared to spend the money to fit an up to date unit .
So are all these guys you read about fitting new autopilots to Skymastes doing so illegally.