Malcolm,
Yes looked at multiple mantra, including this one, but not sure how that is represented in the menuki on the right. The one no the left fits very well to the bonji seed characters, but the one one the right does not. The top does fit a few possible characters. I might be looking at it all wrong, but the bottom of the menuki on the right does not seem to match any bonji that I have seen. The seed syllables for Kan Man appear to be different. If there is an example of that that you have found, If you could post, would be much appreciated. In the above mantra, not sure I have recognized the bonji the fit the menuki. It may just be a artistic style difference in making the menuki fit the size of the other side.
Maybe it is not supposed to match exactly, but everything else I have seen previously appears to match up fairly well with known characters. The Bonji Taikan gave hundreds of additional variations which I have not seen on line, but still not a fit that makes sense to me.
Thanks again for all your help and input. This has been truly enlightening to me, as I have learned a number of other things about bonji and Sanskrit in the process, but this piece still alludes me or my understanding of the symbols.
Nick