Roma Aeterna - operum?

Latin Learning Latin
1

In Cap XLVIII in Roma Aeterna, I find in lines 122/3:
… ut non multum abesset quin opera ac vineae desererenter.

then in line 126:
… ab apparatu operum ac munitionum nihil cessatum (esse).

My confusion is that in the margin to line 123 Orberg has:
opera -um n = opera obsidionis, munitiones

I cannot find the noun opera -um in my search. In line 126, I take operum to be the genitive plural of opus.
Does anyone have an explanation, or is this an error of some type.
Thanks…

2

The marginal “opera -um n” is just giving the plural of opus.

3

I am sorry you had to waste your time on this! I woke up this morning and realized I had tunnel vision and missed it as plural. On top of that, I actually answered my own question. So embarrassing.
:blush: