Hi,
You can use a collection name starting with a number, but the mongo JS shell recognizes that as a beginning a variable rather than a collection name so you need to quote it:
> db["1"].save({"test2":1});
> db["1"].find();
{ "_id" : ObjectId("5003d9c1c27f710e17072683"), "test2" : 1 }
> db.getCollection("1").find()
{ "_id" : ObjectId("5003d9c1c27f710e17072683"), "test2" : 1 }
Cheers,
Stephen