Michael here is another photo with the 2 different tubes if you cant find the steel extender you could trim down the blank to fit,but its probably better you get the part you need
Richard Greenwald sells a R2B Converter made of plastic there only a dollar check out Miscellaneous Pen Parts on his web site, you can find it here in the Vendors pages FIRST click on the LINKS at the top of the home page left side.