although I'm a little late and you solved the problem on your own this information could be helpful.
The S1 byte indicates the clock quality in sdh/sonet networks. You received 0xf (1111) which is known as "don't use for synchronization" in sdh world. So it should be investigated why the connected sdh-equipment sends oxf. Most probably the sdh-equipment will have a clock-problem itself or it is just misconfigured. If you ignore the s1 byte you will clock from a line which should not be used for clocking in sdh world. but I think this won't bring you in trouble because the clock you receive will be good enough for the pos interface.
Further information of how clocking works can be found here:
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