The 525 was introduced in 1969, added pneumatic front tires, and a 3 speed transaxle with live rear pto shaft (used to power pulley/belt rear implements).  The 525 has an 'early' and 'late' variety very few folks consider.  They can be found with the early steel steering wheel, or the later style plastic steering wheel.  The early model (and 515) have a slotted seat pan to allow the use of a 2-speed, planetary reduction for the rear end.  This gave 2 speed ranges to each gear.  The late Serf can utilize that option but the fender/seatpan has to be cut to accommodate the shifter.

The next model was the 535, introduced in 1971.  It included a new styled grill and fender/seatpan and it no longer appeared as a current model by 73, and is presumed to have been available through 72.

All Serf's were 5hp, all were just over 52" long and half as wide.  They used a 28" 2-blade rotary deck and 27" snow thrower that were proprietary to the Serf model alone.  The Serf used other implements shared by the Yeoman and Broadmoor machines.  

42" dozer

36" lawn roller

Trailing leaf vac

26" rear pto powered lawn revitalizer

30" rear mounted tiller

Snowcab

Tow behind 4cu ft and 10cu ft tow carts

40" mid mount grader blade

...and a variety of front and rear weights, hubcaps, and electric start options.