To answer that you need to ask “when does a steel billet become a receiver/body?” A finished Mk1 can’t be converted to Mk3 unless like the Indians, you tack bits on with welds. If it’s just a raw lump that was going to be a Mk1 body today but you want it to be a Mk3 tomorrow then yes you could. I very much doubt there were any Mk1 bodies on the production line that ever became Mk3s. More than likely the last Mk1 rolled of the line as the beginning of the line were re-jigging to the Mk3. And Inglis never made a Mk3 so would never have that issue to consider.