It depends on how original the straight stock was. If it could not be lettered as original, it would be considered just another gun with a straight grip conversion.
Bill: My first Parker was a nice 1920s Trojan 20-bore purchased for $900 in 1974, and it had a straight-grip stock. I knew nothing about Parkers then except that I wanted a 20-bore and preferred a SG.
Soon I had people telling me that it was not original because Peter Johnson said so in his book when, in fact, he simply quoted Parker advertising, which seemed to limit the options on what was considered to be a standard "no frills" hardware store gun. Much later Remington tried to jazz-up the Trojan as a skeet gun and a few were made with straight grips. One was seen at the Vintage Cup two years ago.
As to my SG 20-bore, it had a thicker head of stock which I
now consider endemic to late-Remington Parkers and, having removed the trigger guard, I observed that it had the characteristic stamped serial number in the trough. But this was before I understood the implications of these markers of possible originality. I sold the gun in the early 1990s for $1,150 and the buyer soon became dissatisfied because of the constant advice from nabobs-of-originality that "...Parker never made a SG Trojan."
Using hindsight, what probably happened was that when Remington geared up to make a commercial number Trojan skeet guns, shortly before giving up the Trojan line all-together in 1938, I suspect they turned out an inventory of SG blanks and, sometime thereafter, my Trojan went back to Remington or, more likely, Larry "Babe" DelGrego to be repaired and/or re-stocked...and the owner opted for the SG. In that time frame, Babe constructed a new Trojan 20-bore from Remington/Parker parts for Lawrence's 10th birthday (1975?), which is pictured in the color section after p.178 of
Parker Guns: The "Old Reliable." All things are possible! EDM