I'm replying to my own post, hope someone will find the details helpful.
First of all, TDS5054B does need the Option for 16M (16/8/4) enabled/activated through 'Option Key'. I don't know why Tektronix has mentioned 16M (16/8/4) as default option in product brochure! (I've not received any response from Tektronix yet reg. this issue.)
As I've mentioned in the earlier post, the Option Key Generator did not work for my TDS5054B initially but it was the only viable option left for enabling the 16M Memory Option. I've decided to go through the code base (inline documentation helped a lot.) to find a way out. Even I've compiled a small data set comprising of Model No., Serial No., UID/Instrument ID, Option Key & Option Names from eBay Listings to have a better understanding of the key generation process. And the effort paid off, I could enable the 16M Memory Option (& many other software options) on my TDS5054B.
From reverse engineering, I've found that the existing code does not natively cater for the 'B' variant of TDS5000 series. With the help of the data set I've compiled & with some experimentation I could make it to work for my TDS5054B.
So, this utility works perfectly for TDS5000B series scopes too. I've not modified the code base yet to add support for TDS5000B natively but that won't be difficult (I'm not familiar with Python but the modification needs few lines only & is doable for me.). I may add the 'Augmentation Part' & put it in my github repo. (with the details I've reverse engineered from the Utility.) if it helps the community. In the meantime, if anyone needs help with the Option Key Generation for TDS5000B series, can send PM to me or can join the discussion here on this thread.
I would like to mention that the original work was done by other eevblog forum members,
https://www.eevblog.com/forum/testgear/tek-csa7404-repair-project/msg484817/?topicseen#msg484817, I've just extended the feature set a bit. Without their excellent work it would never have been possible.
Edit: Edited as per the suggestions received.