Since the "terrible forgery" stamp in question mysteriously got vanished, here it is.https://www.ebay.co.uk/itm/203352380099
For the record, this stamp is absolutely genuine in all aspects–the base stamp, the overprint and the cancel.
The stamp is Gwaliyar 1896 2A6P SG 23 with Gupta type 13 barred circular cobra cancel.
The 2A6P denomination was exclusively used on registered letters with ½A/6P being the basic letter postage for weight upto 5.83gm (½ tola), the rate in effect from Jul 1,1885 to Mar 31,1905 and 2A was the registration fee per article from Jul 1,1885 to Apr 23,1922.
As usual, perf12's posts on India should be taken with tons and tons of salt.