Found my Page's...it simply notes the existence of the plug, describing it as "Sealing Wax." Material has been discussed on here, strongly suggesting it was something with a higher melting point. Page could have been guessing, or perhaps just meant "Waxy stuff that seals a hole." And, of course, actual sealing wax has a long history with many different ingredients used.
Page also mentions the need to remove it for service, and fails to mention any need to replace it. Same old question...why was it put there, and if there was a why, why aren't there any notes on replacing it after the tuneup?
And...distributors were made by a number of suppliers including Standard and Autolite. Could the wax have been added by one of them, perhaps one only?
Page confidently states that Model A distributors were made by American Bosch, for what that's worth in this muddle.