Possibly could be Tom. This is an area where I have a 'general' knowledge, rather than a 'specific' knowledge.
I do know that Snyder's (who carries BOTH Model T AND Model A parts) has (page T-78) a "Straight Saddle Arm" for a 1/2" hole Top Saddle (part # T-3876-B) which also is what they list (page A-190) for the '28-'29 Model A. So the question gets down to what bolt diameter and what thread size of the "Top Saddle Stud". For the '28-'29 Model A, Snyder's carries (page A-190) #A-37329-A.
If there is different diameter Stud than what Snyder's carries, I guess you would have to make your own. This is what I had to do in 1960 when there were NO parts houses. Rather than a straight bolt, mine had a 'shoulder' where the treads stopped. At this late date, I no longer remember why it was that mine had a 'shoulder', but I see that the later reproductions do not.
To help me in today's question I once again referred back to my MAFCA/MARC RESTORATION GUIDELINES . There may be something there, but I did not find much. Nothing was listed in the section on TOP (area 9), or rather I should say, I did not find anything there - it may have been there, I just did not see it. There was a reference indicating that "Holddown straps for Phaetons were optional accessories" - so it may be that (by extending that thought) the Saddle Arm and Saddle also did not come with the car, but also was an optional accessory. The GUIDELINES suggest going to area 16 - Accessories.
Area 16 (Accessories) page 16-1, under FORD AUTHORIZED ACCESSORIES (authorized as 'Dealer Supplied' accessories), is listed "Top rest saddles - Standard Phaetons" . I presume that the Saddles and required mounting bolts came as a unit.