We have talked about it before. The first examples of this type of conversion that I was made aware of was for the Kriss firearms.
An original magazine body is used. Just discard the factory base plate and spring. A rigid tube that has equal capacity is added to the bottom of your OEM magazine. A new spring and usually a follower is included in the conversion
HKParts and other on-line vendors offer these for various firearms. The one being shown here in this thread converts a 10 round HK45 magazine to a 20 round extended capacity magazine
This will be the fifth firearm that I have utilized this style conversion on. All of the others have proven 100% reliable in 5 or 6 years of use