Somewhere about 300.000 serial number and higher had the plastic upstop.  Early plastic upstops were very small and then I think there was 2 later versions of size of the orange upstop.  See some pics.  First one is the early style of LF and then two later models with orange upstop.

 

I received a 33 to work on once and they had used a toothbrush handle for the upstop.  The machine was used in a factory so they had the smarts to make it work.

 

I have a few plastic upstops that were fabricated by a fellow Greenkeyer if anyone needs one.

 

73,

 

Wayne

KB1FDW