jdbe,
Williams makes the rear slide/blade sight in a few different heights, might give them a call on their 800-530-9028.
The Fire Sights come in 1/4" and 5/16" heights. The regular WGOS comes in four heights.
3/16" adjustable .320-.420
1/4" adjustable .382-.482
5/16" adjustable .445-.545
3/8" adjustable .507-.607
Hope that helps, I don't know what blade comes standard with the Huntsman/Sidekick but a call to Customer Service (H&R/NEF) might get you somewhere if you know what you are looking for after calling Williams and getting their input. Another option is a shorter front sight, but let Williams know where your gun is shooting and they will be able to help.
I have a 2005 Williams Catalog and price sheet and all of their sights ARE NOT on their website. Request a catalog and it will be there in less than a week. The H&R/NEF Customer Service did give you the correct code #17778 for the WGRS-H&R Peep Sight. You may or may not need a new front sight, but Williams should be able to tell you that. It says for the H&R Buffalo Classic to use a .570W front sight.