Would NOT add knobs until the piece is THOROUGHLY checked out. If even remotely valuable.. and an antique... I wouldn't do ANYTHING to it, especially the finish.

No knobs....use the key.
And American Empire style is about 100 years earlier than 1910....well not quite 100 years but almost.
Use the key...I have several old cupboards that don't have a knob....I use the key. I figure that it was always meant to be locked.
Linda C

No knobs it is, will try and post photos after weekend Thank you

I have a knobless armoire I was able to use a knob on without doing any harm--the keyhole goes straight through and is large enough to hold a screw. If you can use a knob without damaging the piece--keep in mind how the base of knob and screw meet the bookcase because there is a good potential for marring--I'd say it's okay. But I wouldn't do anything that could damage it or be irreversible. You're probably stuck with the key. :)

It depends a bit on condition and value too. And your plans for the piece. I take exception sometimes to having to suffer for the purity of the piece if it means 60 years of compromise for me. If I need a knob, or a shelf, in a piece, I need a knob. Some pieces I wouldn't dream of taking a drill to. Others, I admit, I have. And would do again. After all, if you need a knob, so might the next owner. Just do it well.

Karin L

A word to the wise. The old keys sometimes get broken in the lock, especially if the wood swells in summer. If you have only one key, get a smith to duplicate it, or several of them. I had it happen to me on an old china cupboard and was fortunate enough to get it out with a forceps and had a repro key made. The smith warned me to get several made then and says it happens a lot.

Calliope that was one of my worries, the key is very rusty.

It was just as much my fault as that of the key. I live in a very old house and it gets humid in summer. The wood in the doors stick, so I just yanked and pow. I know enough now to gently rap on the doors and then turn the key. But since one turns the key so it doesn't slip out of the keyhole, the key itself comes under stress because it becomes a handle and the cleft part of it just fatigues. It can get very nasty to get it out, and may mean removing the doors and door hinges and oftentimes their access is on the inside. Don't want to ruin the integrity of an antique for want of a cheap key. I think the smith only charged me $2. This has been years ago, and thank God we had a real locksmith in a nearby village.

I have an antique English hanging corner cupboard. When I bought it the key that was used to open the door wasn't the proper key to work the lock, it was just a key that fit into the hole and used to open the door. Well my house must be on a slant or something because the door wouldn't stay closed without locking it....so I called a locksmith and he came and made me a new key that locked the door.
My point is....rather than risking a rusty key breaking in the lock....most times any old key will work to open the door.
Then there was my mother who locked her desk (not an antique) and misplaced the key.....there was a panic for a week until she found it.
Have another key made....it's worth the small cost.
Linda C

Yes it's worth the cost, and what you do when you get a repro made, is take the original key and secure it somewhere where you can find it then use the new key. I had all sorts of old keys around and none would fit the brass key to my china closet. They're often not interchangeable the older the piece gets and often made of a soft material like brass and they can bend and distort and impact the workings of your locks.