Summer, I have not knitted this pattern but, because I am intrigued by it, I am looking at it right now.
For an experienced knitter, who is good at 'reading' their knitting, I don't think it would create too much of a problem. However, for someone less experienced, it could be rather difficult.
I have over 50 years experience in knitting, and I am fairly sure I can work out how this is done, so here is my interpretation.
Firstly, once you have completed the M.1 chart (including the M.1B chart) you should have 129 sts, made up of 8 repeats of 16 sts = 128sts - plus the 1 stockinette stitch, for a total of 129 sts.
Now, it says continue by repeating M.1B chart vertically until desired length is reached (forget the lower part of the M.1 chart now, and continue to keep repeating the M.1B chart). I believe this is where it becomes a little tricky because if you continue to repeat the M1.B chart as written, the pattern will not work out correctly, because of the continuing increases as you work your way up the chart each time.
I am assuming you have placed the stitch markers after each pattern repeat because you will certainly need them.
As you keep repeating the M.1B chart you will need to make sure you keep the decreases, in the center of the chart, directly above each other in each repeat, and the increases will need to be worked right next to the stitch markers. This will mean, that as you progress, the number of stockinette stitches between the increases and decreases will get more and more.
For example, after you have completed the M.1 chart to the top (where you have 129 sts) and you continue from there, beginning with the first row of the M.1B chart, which is a purl row, the first 3 rows will be worked the same as you worked them previously except you will be working them over 16sts instead of 12 - e.g. Row 2 you will (K2tog, YO) 8 times.
When you get to Row 4 of M.1B you will work each repeat thus:
K 2 sts in 1 st, K5, K2 tog, slip 1 st as if to K, K 1, psso, K5, K 2 sts in 1 st.
Row 6 of M.1B will be: (K 2 sts in 1st) twice, K4, K2 tog, slip 1 st as if to K, K 1, psso, K4, (K 2 sts in 1st) twice
After Row 6 you will have 18sts in each pattern repeat, between your markers, so the number of stockinette stitches between the increases and decreases, will increase, also, as you continue to work your way through the shawl.
What you will need to make sure of is that in each repeat, when working the right side rows, you always work the decreases directly above each other, and that the increases are always worked right next to the markers.
i.e., in Rows 4 and 8 of M.1B - where you K 2 sts into 1 st, this will always be worked in the st right next to the marker.
In Rows 6 and 10 of M.1B - (K 2 sts into 1 st) twice, will always be worked in the 2 sts next to the marker.
In this way the number of stockinette stitches between your increases and decreases will continue to get more and more, as you go.
I hope you can follow this and that I have not confused you more.
