In Asraare' Aasaar (volume 2, page 7), Fadil Mazandarani quotes Ali Mohammad Shirazi from his book - Tafseere' Surah Kausar:

'Then know that surely for him are two Ghaybat which are with Allah's permission. Some people have got the opportunity to meet him in this period, the length of which none knows save Allah. And surely in Ghaybate' Sughra he had reliable agents and proximate deputies. Their period of deputation lasted for 74 years. His deputies, may my soul be sacrificed for him, in that period were - Usman bin Saeed Amri, then his son - Abu Jafar Mohammad bin Usman, then the trusted Shaikh - Shaikh Abul Qasim Husain bin Rauh, then Ali bin Muhammad Saymuri. All these were the ones who in Ghaybate' Sughra ordered the Shias towards religious commands and forbid them from its prohibitions.'

Conclusion
Here are some deductions from the above statement of the Bab:

1. Bab confesses to Imam Mahdi's Ghaybat, which for the Shias has always been a differentiating factor between the true Mahdi and an imposter. This means that a person who was always apparent, was generally a recognised figure in society talked and move about freely could not have been the awaited Imam Mahdi. This confession of Bab disqualifies him completely from the title of Imam Mahdi which he was to make only a few years later.
   
   

2. The number of Ghaybat for Imam Mahdi are two in number. Bab never had one Ghaybat let alone two.
   
   

3. One Ghaybat was longer than the other one. The first one, Ghaybate Sughra lasted for 74 years. Incidentally Bab has got his numbers wrong - the first Ghaybat began in 260 AH with the death of the 11th Imam - Imam Hasan Askari (as) and continued till 329 AH, which makes it 69 years. However if one calculates the commencement of Ghaybat from the birth of Imam Mahdi in 255 AH, then the number of years is 74 as mentioned by the Bab.
   
   

4. The second Ghaybat which was when Bab was born is particularly prolonged the length of which none knows save Allah.
   
   

5. Some individuals got the honour to meet Imam Mahdi in Ghaybat. This means that meeting Imam Mahdi in Ghaybat was possible depending on when Imam Mahdi wanted else it would not have been a source of honour for that individual. Obviously, Bab never got that opportunity. If he had, he would surely have mentioned it in his confession to show his special affiliation with Imam Mahdi. This is particularly true since making empty claims was nothing new for him.
   
   

6. Bab painstakingly lists down the four special deputies in the shorter Ghaybat viz
   a. Usman bin Saeed Amri,
   b. Abu Jafar Mohammad bin Usman (son of the first deputy),
   c. The trusted Shaikh - Shaikh Abul Qasim Husain bin Rauh,
   d. Ali bin Muhammad Semori
   
   

7. The 4 deputies in the shorter Ghaybat informed the people about the commands and prohibitions as outlined by Islam, not some new religion or order of the Imam Mahdi. This is significant because when Bab claimed to be the Imam Mahdi at a later stage he introduced a new book and a new order distinct from Islam and invited the people towards it.
   
   

8. Very significantly Bab expresses his desire to sacrifice his soul on the special deputies. If Bab indeed was the spirit/soul of Imam Mahdi, it's puzzling why he would want to sacrifice his soul on his own deputies, should it not have been the other way around?