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Fred the only thing stamped on the arm is the serial no and ducks foot above the scale an H Huges and son ltd below the scale, i posted a photo. I have checked perpendicularity and side error both ok index error is 0.2 m. I have   further queries.the telescope collar is axiallybadjustable, what is the porposebfor this, presumably to centre the eyepiece, is this critical. More worryimg is that the housing for this adjustment is attached byb3 small screws. I noticed these were not secure allowing a small ammount of radial movement. I am assumimg that this is not critical otherwise this mounting method would not have been adopted. As i am measuring an angle the angle will be the same irrespective of the angle of the eyepiece. Am i correct in this assumption. Finally i taught myself sextant use and have always taken sun sights with the star sight.having taken on board the comments on this forum i have tried the l4 wire eyepice and find it somewhat difficult to use as everything is less clear ( more difficult)  is this just me?