Rich
Just to warn you, yes you could use a Eureka F&G HEPA Cloth bag in your Kirby, it would work. BUT, unfortunately, these F&G paper bag conversions that vac shops used to do will SEVERELY cut your Kirby's airflow and cleaning performance!
You may not notice it now when you vacuum with it....but go ahead and put a Heritage II Mini Em-tor, Heritage II outer bag, fill tube and Kirby HEPA Cloth bag on it, and just watch how much better the airflow is and it's cleaning. I would strongly suggest doing that over finding Eureka F&G HEPA Cloth bags for the bag conversion.
Also, just so you know, I think that lady was feeding you a line when she said it was sent into Kirby for rebuilding 4 times. Look at the underside, the label will betray the truth - if the vacuum has been rebuilt, the original makers plate will be removed & replaced with a newer one that looks much different. Also, Kirby would have never supplied a Eureka F&G bag conversion, what they would have done is put a new black shakeout bag on it and put black trim on it. Since it has Dual Sanitronic 50 brown trim on it, chances are good that it's either: 1. A Dual Sanitronic 50 with a Eureka F&G bag conversion, or 2. The lady brought her Kirby 519 into and had a vac shop replace the trim and bag with the bag conversion. And that's assuming you believe that she did indeed buy new - she could have very well bought this Kirby rebuilt like this to sell from a local vac shop. So unless this lady that gave you the Kirby can produce some documentation that says otherwise, I would seriously be doubting her story about it being sent to the Kirby factory and being rebuilt 4 times.