Harry --
You sound like a knowledgeable collector, too! I also have the 15C, 16C, 41CV, and 42S, as does Valentin and many others here.
I'll echo Valentin's comment about the Advantage Pac being the best one made for the 41 -- certainly the best one made by HP. (I have one, too.) Expect to pay > $40 on eBay for one. There are some details regarding the Advantage module that should be pointed out:
1. The complex-number support is the RPN-code routines originally developed for the Math Pac. It is not nearly as complete and easy-to-use as the complex-number functionality in the 15C and 42S.
2. The binary/octal/hexadecimal conversions, while micro-coded, are somewhat cumbersome and designed for unsigned integers only. Again, no match for the 42S/32S/20S, etc...
3. The curve fitting routines, I believe, are the ones in the Stat module you have.
Regarding complex-number functions, the 42S would be closest to ideal if its "COMPLEX" command brought up the two-level menus of the HP-28C "CMPLX" and 48G "CMPL" functions.
The 28C/S is also quite good, but it doesn't allow computations in polar mode or display polar-mode values. Conversions must be perfomed, 15C-like.
Regarding complex-number support on RPL machines:
1. Does anyone know how to enter a complex-valued number in polar form on a 48G or 49G? I'm stumped! Rectangular-coordinate entry is assumed, even in "POLAR" mode (R < Z annunciator); there seems to be no P->R conversion on RPL machines (28C, 48G, 49G) that works on two real-valued arguments.
2. Try this on a 49G in "FIX 5" display mode :
(ALPHA)
CMPLX
(ENTER)
I get the following pop-up menu:
0. MAIN MENU
1. 00000RE
2. 00000IM
3. 00000CONJ
4. 00000ARG
5. 00000ABS
6. 00000i
In addition to the bug, it's not even the same menu as the one provided by option 9 under the "MTH" function. What a POS! I assume that there's a downloadable ROM upgrade, but this product is beyond patches...