Those older drum objective B & L stereos go back , possibly into the 20's. Here is a pdf of a catalogue that covers the SK instruments as they were sold in the 30's.
https://vademecummicroscope.files.wordp ... e-text.pdf
The basic Spencer Greenough stand that they catalogued for about 30 years first existed as black instruments but by the early 50's the colour of the entire Greenough model line was changed to grey and they had upgraded the eyepieces to flatter W.F. versions,in order to keep the model current and useful as a lower cost stereo type, for markets that needed a lower cost high quality stereo. By the 1950's, they were making the Cycloptic apochromatic microscope. I don't think there was a grey model 29, only in black( made pre-W.W. II or during).
The 29LF was only catalogued briefly, having been specifically developed for Dr. T.H. Morgan for his work on Drosophila. The basic microscope is the same body as the model 28 but with slightly differing dimensions of the mounting board, pillar and articulating arm. The model only existed for a few years, and was very close to the 28.
The standard microscope of any model body in the Greenough line was designated by the number 22 to 28, with the 29 existing only briefly. The L designates an inclined head, as opposed to a perpendicular head, The F designates a rotating 3 piece nosepiece, as opposed to a single objective mount.
The 29LF, would therefore be a # 29 body attached to an articulating arm that extends 175mm, on a focusing pillar 75mm high. It would have come with two sets of paired W.F. eyepieces 9 and 12X and three sets of paired objectives, 1X,2X and 3X, in a revolving nosepiece. It would have been mounted on a board of 200 x 325mm. Paired eyepieces of 15X and 18X ,as well as paired objectives of .7X, 4X, 6X and 8X were also available, optionally. It was not on a boom stand, the designating difference of the model in fact being determined by the method of fitting the focusing block on a swivel mount to the mounting board, which are not part of a boom stand microscope.
A similar microscope body, head and objective carrier mounted to a boom stand would be a model 23LF. There was no articulating arm fitted, because due to the existence of the boom stand and arm, it's function would have been duplicated.
In the 1939 catalogue which profiles the 29LF as well as the entire Greenough line, there are 48 different permutations of Spencer's basic Greenough stand, as well as 3 junior Greenough stereos.
....
