You're right, matching a specific magazine is very difficult. When it happens which is rare, a lot of luck is involved. For your Toriimatsu Type VIII magazine, you will also need to look for the correct number size in addition to the number itself. The below image shows the progression of magazine number size changes in Toriimatsu production. Your gun should have the small 2.0mm numbers neatly stamped as is shown in the left magazine. The mid-size 2.5mm numbers started showing-up in 19.5, and the largest 3.0mm numbers appeared around 20.6. There is some overlap in both number size change introduction periods, and some magazines also have both size numbers.