From the braysa in the middle of 58b which states "Tamid kol ha'shana kula kariv k'hilchaso - Nishchat Bishmohen Umechtza . . ." it appears that the gemara never fully abandoned its original drasha that "Bein Ha'arbaim" means exactly in the middle of the afternoon from 8.5 to 9.5 hours of the day but rather revised this requirement from being mandatory to being the preferred time. Therefore, "K'hilchaso" refers to this preferred time specifically even though the gemara has already stated that the only mandatory requirement is that the Tamid be brought from 6.5 hours and on. Otherwise, why refer to this time used all year round as "K'hilchaso". Is this correct?
Aharon Levine, Baltimore, MD
The Gemara in fact did abandon its original D'rasha that "Bein ha'Arabyim" means exactly in the middle of the afternoon for a different meaning, from when the shade of evening starts (six and a half). The reason that the Tamid is usually brought later is not due to a Gezeiras ha'Kasuv, but rather to the logistical advantage of being able to Shecht more Korbanos during the day (see Rashi DH "Ela Amar Rava" and Rambam Temidin u'Musafin 1:3).
All the best,
Yaakov Montrose