I am very interested in this posting. I have learned Yang style Tai Chi from a Grandmaster in London and his postures are almost identical in all but a few small details to Yang Zhen Duo's postures in his book Yang Style Taijiquan, published in Beijing. I have a number of books on Yang style Chi including 'Tai-Chi Chuan Its Effects and Practical Applications' by Yearning K. Chen, published in 1947. Chen was taught by Tien Sou-Lin. All of the line drawings in this book are obviously based on the famours Yang Cheng photos as can now be seen in Mr Swaim's recent translation of Yang Chengfu's 'The Essence and applications of Taijiquan'. However, there are two main differences: the opening, and Grasp Bird's Tail. Perhaps Yearning K Chen's book, although of relatively late date, might supply the 'missing link' as far as the opening movements are concerned. The entire text for the opening movements is as follows:
"1. Commencement of Tai-chi Ch'uan (T'ai-chi Ch'uan Ch'i Shih)
Stand upright facing north. Separate the two feet at a distance approximately equal to that between the shoulders. Put the two hands beside the thighs with the palms turned downward and the fingers pointing forward. Put the head straight and set the eyes to the front. Keep your whole body loose, to make every part natural and at ease. Raise the two hands before you gradually without exerting any force up to the height of the shoulders. Bend your knees and lower your body, and at the same time draw your right hand back to the front of the chest, your left hand remains in its former position, then turn the upper part of your body to the left, stretch your right hand out northeast, and draw your left hand to the back to the front of the chest with the palm turned upward. Turn your right hand to the left. When it is about to reach your left hand, lower your left hand and cause it to make a horizontal circle counter-clockwise. Turn your left palm upward again. When your hands are before the left side of your body, the left one is under the right and they appear as if they were carrying something. Turn you hands to the right in front of your right side. Cause the two hands to change positions, that is to say, lower your right hand and cause it to make a horizontal circle clockwise. Turn the right palm upward and the left palm downward, the left hand up and the right hand down. Turn the hands to the left in front of the left side (the position of the hands is unchanged). Shift them to the central front. Lower them and cause them to make two and a half horizontal circles clockwise, making the circles smaller and smaller and lower and lower. Simultaneously lower your body and rest on your legs. Put your right hand under your left elbow. Turn the heel of your left foot to the left. the centre of gravity is shifted to the left foot."
The Grasp Bird's Tail posture is then in two parts, firstly beginning by stepping out to the southeast and warding off with the right hand, and then reversing the posture to face to the northeast. Otherwise, the form is the same as Yang Cheng Fu's and Yang Zhen Duo's.
Clearly, the opening movements differ radically from the Yang Cheng Fu raising and lowering arms movement with which we are all familiar, and are considerably more involved. Perhaps, as I've said, this book provides the link you are after. Kind regards, Simon.