Can anyone tell me the MODEL and BARREL LENGTH of the S&W .38 revolver used by Lee H. Oswald to shoot officer Tippets in Dallas ?

It was a chopped S&W British Victory. Originally a WWII service revolver used by the British Commonwealth, and chambered in .38 S&W, and with a 5" barrel. During the 1950s and 1960s, the British surplussed out HUGE numbers of them which were imported into the USA by numerous American surplus arms dealers. A great many of them were rechambered to .38 Special and had their barrels shortened to 2" to sell better on the US market. And one of those is what Oswald allegedly used to kill Officer J. D. Tippitt. If you Google "Oswald's Revolver" you will probably find everything known about it.

A previous posting here: Lee Harvey Oswald's S &W 'Victory' Snub Nose

The thread that DWalt linked is good, but lacking pictures.

I posted this thread a few months ago:

Oswald's Victory BSR: The basics