Lots of people have single shot rifles [H&R] with exposed hammers. I had one during the 70's.
In my 2005 GRAF & SONS catalog they list a "Hammer Extention", made by Uncle Mike: $7.99
UM 2456...Ruger Blackhawk, H&R Topper, New England Arm's Handi Rifle.

FWIW, the NEF/H&Rs come with a hammer spur extension, I don't use em unless I have to....I have an Uncle Mike's on my BLR, they both work fine.

I'm sorry if I am wrong.
About 8 + months ago somebody with an H&R was complaining about the hammer. I told him about the extention but, could not tell him who made it or where to get the Hammer Extention.

The H&R spur extension can be mounted left or right side....one comes with every new rifle or they can be purchased separately....

http://hr1871.com/store/index.htm