I'm not sure that there is a documented reason (I have read most everything I can about these conversions). So what we have left is speculation
Weight?
Easier to "port" by removing the shield?
I believe that all of the NMN's were converted after the NMA's (if I am wrong here steo in, please) so maybe they were trying to speed up the process..............
Obviously the 38 cartridge in use was not so powerful that this removal caused problems.
Interesting question though!