It was not you who flaunted your alleged expertise, it was other participants in the thread.
I've only mentioned that I'm a lawyer and law professor. If others have repeated that point, they have not been flaunting my expertise. In any case, I don't claim expertise in commercial transactions, banking laws, or the Federal Reserve system. I have only stated that I'm familiar with the way legal language signifies, and that "should" is not a word typically used in law to denote a legal requirement, failure to comply with which triggers sanctions.
But you're the one with the burden to establish the legal significance of the language and its effect, if any, on Oswald's postal money order. I raise the following points as issues I think you will have to consider as you pursue your proof-gathering:
1. Circular No. 4928 was not a part of the "federal regulations," as you have claimed. The Code of Federal Regulations (CFR) contains regulations that have been proposed by federal agencies, submitted to a notice, publication, and public comment process, and eventually promulgated as rules in the CFR.
2. Federal Reserve operating circulars, of which No. 4928 was one, were different. They were not put through the regulatory process I just described. Instead, they were issued by Federal Reserve Banks to their participating clearing banks, and the duties they created for banks were contractual in nature, not regulatory in the typical way. Because these circulars were, in effect, banks talking to banks, this explains some of the nonbinding "should," "may," or "banks are urged" language found in No. 4928. (You should read the whole circular to get a feel for how "should" operates in the context of other wording.) These documents, in addition to occasional mandatory language, were setting goals, aspirational norms, and best practices for members.
3. It is worth asking whether Circular No. 4928 actually applied to Oswald's transaction at all. Oswald's money order was processed by the First National Bank of Chicago, apparently through the Federal Reserve Bank of Chicago (District 7). Circular No. 4928 was issued by the Federal Reserve Bank of New York (District 2) to its clearing banks. Since these circulars operated as contractual understandings, could a District 2 circular bind banks in District 7?
4. Once again, I'll suggest that "should" is different from "shall" or "must." It is advisory or recommendatory in tone rather than mandatory. "Should" implies a scenario in which existing practice has been observed to be uneven. "You should brush your teeth daily" suggests that the child is inconsistent. Failure to comply with a "should" would not likely trigger penalties.
5. Let's say arguendo that you can prove that "should" was mandatory and that Oswald's money order lacked required stamps. It appears that the money order was honored anyway: Klein's was paid, and Oswald, as Hiddell, received his purchase. Bureaucracies often operate that way. We want our banking system to function and not be held up by the venial sins of bureaucrats.
6. If you think that the absence of stamps resulted from a bungled conspiratorial fabrication, you still have to prove the fabrication and the cover-up. Oswald's money order by itself does not do all that work for you. This is why I say that you have all your work in front of you.