Mark XV or Mark XII

Forgive my ignorance, but I've been looking at these two models and can't tell them apart other than a slight variation in size.
To me they appear identical in appearance so please would someone be kind enough to explain the differences to me?
Thank you.

There is a significant difference between the movements.

Mark XII => JLC based manufactur calibre.

Mark XV => ETA based calibre.

Ah,
Many thanks Hebe.
I now have some reading to do !

While the JLC caliber may be the more unique, the ETA caliber might be the more robust and reliable. I once spoke to an IWC watchmaker about the rather thin JLC movement. He said, this caliber is not exactly service-friendly because of its very thin layout. In fact, when a Mark XII comes in for service, some collegues spontaneously want to go on holiday... :-)

Time by time this story comes up.

At that time IWC has had no own flat movements and still has none.

Most high grade manufactories have flat movements since many years. JLC offers this feature since many decades.

I think that story has been told, because the IWC watchmakers have had no experiance with such flat movements. All IWC inhouse cal. of that era were indeed much higher.

An example from a Jura watch museum.