I find that hard to believe. Here are excerpts taken from http://www.mindef.gov.sg/safti/pointer/back/monograp/mono_cld/CLDmono.pdf
“On 1st June 1966, the first batch of officer cadets reported to SAFTI for training. These 300 men were selected from 2,500 applicants. All had volunteered; we had no National Service then. Like Singapore then, our armed forces were in their infancy. We had no tradition of military service, and those who joined the armed forces enjoyed few incentives or perks. But these men had the pioneering spirit. They savoured the sense of adventure that comes from being the first, of doing their best and blazing a trail for others to follow. They were training to be leaders of the men to defend our new republic.”8 Prime Minister Lee Kuan Yew, 1990.
"Hence, the urgent effort by the early Singapore Government to introduce National Service during the period from 1966 to 1967. In November 1966, Dr Goh Keng Swee hinted at the Government’s decision to adopt National Service when he announced that all new government and statutory board employees had to undergo military training. In February 1967, PM Lee announced the government’s intention to introduce National Service, and a “National Service (Amendment) Bill” was introduced in Parliament for debate in the same month. From 28 March to 18 April 67, some 9,000 eligible men were registered for National Service. By August 1967, the first batch of 900 men was enlisted to serve in the SAF."
"A significant event that preceded the enlistment of the first batch of national servicemen in August 1967 was the promulgation of an SAF Code of Conduct (COC) and a public Declaration on 14 July 67 that spelled out the relationship between the SAF and Singapore society."
You are saying something different from what the above which are official records. As far as I know, it was only after 1967 that NS was implemented. The first batch of 300 men were all volunteers, and this can be confirmed easily as most of the 1st batch officers are still around.
To attest this, all recruits were required to undergo 3 months basic training and another 3 months Section Leaders training before OCT which is another 6 months training. Therefore, if the first intake of NSmen was in July 1967, then the earliest they could join OCT will be in December 1967, and their POP therefore will be in June 1968. That will mean at least 4th Batch OCT.