They needed to get under .30 for B free flight.  A lot of engine makers, including Fox, K&B, Supertigre, and even Nelson, built 29's out of downsized 35's and 40's.

How can you tell an Mk series?  I have a Fox 29 ... I assumed it was a stunt engine.  Ran it for awhile in a Vampire, but it had a horrible burp that I couldn't get rid of.

Don, we sorta talked about this at H'vile.  Fox made 29 versions of several versions of their engines.  The Fox Stunt 35 case was also built as a 29.  I had one for awhile.  The next one, the first combat case with the half inch crank also had a 29 in at least one of the versions, plain head, black head, and red head.  Then they built a 29 sleeve in the M IV case, I had one around here.  And finally they built a 29 version of the Mk VII that you showed me at H'ville.

As far as I know all the different series were more or less interchageable.  Thte stunt 29 I had may have had different timing.  It liked to run faster on a 9/6 and more in a rich 2cycle but seemed to fly as fast and run as well as a regular stunt 35 in the same plane.