Mick , its to stop down the main 102mm objective so not as much light reaches the eyepiece , not really much use in an 102mm scope ( this is used mainly in large reflectors as Jupiter in a 16 incher will almost blind you ) but some use it mainly for solar viewing , turning it from a f10 102mm scope to a f20 50mm scope as some Ha solar filters work best at above f15 , the ' Quark ' for one .
On your mod's you will feel a difference and don't forget some pipe/ wood packers in the tripod top where the legs bolt on , in between the casting .
Looking forward to lots of photo's .
Brian.