Complex sentence: Only a few test machines were ever fitted for the engines; some additional airframes with the enlarged wing, were slated to fly with high-altitude versions of the standard DB 603 powerplants, but only one or two test machines flew with them.
Split into simpler sentences: Only a few test machines were ever fitted for the engines; some additional airframes with the enlarged wing,. These examples were slated to fly with high-altitude versions of the standard DB 603 powerplants, but only one or two test machines flew with them.