Hey Hank's

 I know exactly what you're looking for, unfortunately, I personally do not have one.

 There was a U.K. importer doing business in Front Royal, Virginia named John Appleton, and he sold not only Martini actions, rifles, parts; but also, shooting accessories such as P-H sights and the alum. blocks you are wanting.

 Mr. Appleton has gone to be with the Lord (hopefully), but I do see that his web page is still active.  He had quite a large inventory of British firearms, predominantly BSA/Martini, so perhaps someone in his family is either liquidating the inventory in his absence, or continuing with the business.

 If you will goggle "John Appleton Guns" you will come to the web site to which I refer.  I hope someone can look through what is left at the old shop and get back with you with the loading block that you desire.

 Depending on how capably you may be, it would not take much time or materials to make one up for yourself.

     
  Creedmoormatch