In my experience, the most commonly-overlooked options are different spacer colors/materials, single hilt for Randalls that come standard with a double hilt, single hilt with a squared top, thumb notches, and a radiused butt cap. In the case of spacers, brass/black spacers match brass hilts and butt caps, and aluminum/black spacers match nickel silver hilts and butt caps. All-black spacers are also possible (and non-chargeable, because there is no metal). Note that red is another color that can be specified for use in spacers, in lieu of black. In other words, it is possible to specify all-red, brass/red, and aluminum/red spacers.
The following photo shows brass/black spacers (with a brass hilt and radiused brass butt cap):
The following photo shows aluminum/black spacers (with a nickel silver hilt and radiused duralumin butt cap):
The following photo shows all-black spacers (with a brass hilt and brass butt cap):
The following photos show a Nordic Special (Nordic Knives' Dealer Special) with its standard forward curved double hilt, along with one with a single hilt (with squared top and thumb notches):
The following photo shows a single hilt with a squared top and thumb notches:
The following photo shows a radiused butt cap (the hilt and butt cap are fine silver, and the spacers are sterling silver):
Note that some models include metal (aluminum or brass) spacers as standard, such as the #25 (or when specifying the #25 handle) and #27. For these models, the hilt and butt cap/plate materials determine the spacer colors.