Dietmar --
No. SHOW displays the 12 digits of the mantissa, which start with the first non-zero digit -- not 12 decimal digits. Unless something is different about the HP-35s from the HP-32SII and HP-33s, 123.345675346246 can't be entered.
As for the decimal point, it seems that there was a simple rule for all mid-grade Pioneer models: SHOW will display the decimal point if possible; that is, if the mantissa is of a magnitude such that the actual value can be displayed without an exponent.
High-end Pioneer models can show the full 12-digit mantissa and exponent, so do not need to omit the decimal point with SHOW.
Spice-series, Voyager-series, and low-end Pioneers with their 7-segment LCD's do not show the decimal point with, respectively, MANT, CLEAR PREFIX, or SHOW.
The HP-41 didn't have the mantissa-displaying function.
-- KS
Edited: 31 July 2007, 3:00 a.m. after one or more responses were posted