Macomber produced many, many different products over the years with a variety of designations and even some with different safety factors. There was not just "one" catalog covering all of their products, there were many, many catalogs. Their products, generally, did not follow SJI specs until they started producing J, H and LH's in the mid to late 60's.
I would suggest that you make a site visit and obtain dimensional data on the chords and webs and obtain a joist tag with a designation. This will help, then, determine the type of joist that you are dealing with.
