Mongoimport of csv file seems to produce odd documents in collection

Hi there - I’m new to programming and Meteor, but have worked through Discover Meteor and am trying some things out.

I would like to import a lot of data from a Google sheet into a Mongo collection - I found:

which seemed to point me in the right direction.
I created a new collection CardSets then added a test record though fixtures.js as I’d learned. These appear fine. Then I prepared a .csv file and followed instructions for mongoimport:

mongoimport -h localhost:8081 --db meteor --collection cardSets --type csv --file test.csv --fields set,eng,esp

The import appeared to work ok and imported my records.
On inspection with Mongol however I found that the imported records had an odd looking _id

When trying to manipulate these imported records in Mongol (duplicate/delete) it threw and error whereas it had no problems with the test record I’d entered. Clearly this is a problem but I can’t find any information to get me any further.

Are any of you using mongoimport without a problem and could shed some light on this for me?
or else suggest another way to import my data?

Any help gratefully appreciated!