Yes there are definatley no graphite lines on the back.
From the scan I too cannot make out any watermarks. The thickness of the paper makes me think it is a later issue. Looking at my earlier examples they often are more opaque from the reverse and the watermark general clear.
The two Â½d I have shown are multiple crown which can just be made out on the first stamp. The second shows the thinner paper. I was going to start a thread on these because of the difficulty I had when I got a box of about 20,000 Wildings.
Back to yours. Try holding yours up to a bright light source and look through it at the areas without ink around the bust in the oval or in the border between the stamps, it may well show up. (edit) It may be inverted or sideways so rotate as necessary.
Since this is one of the thinner papers you really need to check the phosphor bands on the front. Put them on the palm of your hand and hold it up to daylight so you are looking across the flat of the stamp. The bands should stand out because of the difference in refraction.
Hope this helps.