The applied in the Sea Post marking of registered (as in the OP's and Steve's covers) or Sea Post Due marking are difficult and may yield new finds. Even in the present time, this far removed, new finds such as in the OP can be made.
Additionally I see no manuscript notations on the OP cover which would indicate alteration by a collector. Even the manuscript 4 is fully in keeping with the other aspects of the cover as well as a proper addition. On the reverse, the poorly erased pencil text seems to have been done solely to correct the return address. Since mailed on board, it needed to be returned on board if an issue arose before landing.
As my SCF name implies, I chase parcel post and have done so since 1983. During that time I had contact with other serious students of the issue. During those years I found sea post items which were a surprise. The first, a Japan-Shanghai sea post posted on board cover with US parcel post stamp entering the US mails in Shanghai. Even Michael Karen did not believe such could exist until he had it in hand. Around the same time a Sea Post Due marking in cover surfaced with parcel post postage due stamps. And lastly, just last month a US-French Sea Post parcel post usage surfaced. That is the first such example with parcel post I have seen, nor was such in Doolittle's holdings, nor known by Gobie, Schlesinger, Bothamley or Ward when I personally spoke with them over the years about sea post material. New material can and does surface.
Here is the US-French find:
[Hello Steve. I am still hoping you get that discovery cover to sell to me as previously discussed. Dan