In regards to 1968 Silverface Twin Reverb there was ONE speaker type used for this amp model for 67 through 81 production years:
The Chassis Serial number: A14690 is appropriate for 1968 production year. The codes on the Tube Chart will allow you to work out the specific production month. I am unable to read the information from the eBay Tube Chart photo but I expect one you receive the amp there will be no issues. This "drip edge" version of the first production period of the Silverface is the most desirable for collectors and returns a premium value. Silverface Twins are currently undervalued and I expect at some near point prices should start heading back up. SF Twins are a great amp.
Power Transformer Code
It hard to read in the eBay photo but appears it might be
1967 production year in 27th week.
Output Transformer Code:
It was made in 40th week of 1967
The Silverface Twin Reverb Output Transformer used 4 ohm taps for the Internal dual speaker pair and the External Speaker Jack.